diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8d5338bb..fa860566 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,8 +5,23 @@ All notable changes to this project will be documented in this file. This projec
 
 ## Next Major Release
 
+### Added
+
+- **BREAKING**: Added `code()` method to the `ListOfErrors` interface. This returns the first error code in the list, or
+  `null` if there are no error codes.
+
 ### Changed
 
+- **BREAKING**: The constructor argument order for the `Error` class has changed. The new order is `code`, `message`,
+  `key`. Previously it was `key`, `message`, `code` because code was added later. However, in most instances `code` is
+  the most important property as it is used to programmatically detect specific error scenarios. This hopefully will not
+  be breaking, as the docs specified that named arguments should be used when constructing error objects.
+- **BREAKING** The error and error list interfaces now accept `UnitEnum` instead of `BackedEnum` for error codes.
+  Although technically breaking, this will only affect your implementation if you have implemented these interfaces. All
+  concrete classes provided by this package have been updated.
+- **BREAKING**: The key of an error can now be a enum - previously only strings were accepted. This is only breaking if
+  you have implemented the interface yourself.
+- Updated `KeyedSetOfErrors` to handle error keys now being strings or enums.
 - **BREAKING**: The `Guid::make()` method will now convert a string that is a UUID to a UUID GUID. Previously it would
   use a string id.
 - **BREAKING**: Updated the `GuidTypeMap` class so that it now supports enum aliases, enum types, and UUID identifiers.
diff --git a/docs/guide/toolkit/results.md b/docs/guide/toolkit/results.md
index a90cdf9d..92b2f042 100644
--- a/docs/guide/toolkit/results.md
+++ b/docs/guide/toolkit/results.md
@@ -125,12 +125,12 @@ Alternatively, you can provide the `failed()` method with an error object or a l
 
 The error object provided by this package has the following properties:
 
+- `code`: an enum that identifies the error and can be used to programmatically detect and handle specific errors. It
+  is intentionally an enum, because to programmatically detect specific errors, the codes need to be a defined list of
+  values.
 - `message`: a string message that describes the error.
-- `code`: a backed enum that identifies error and can be used to programmatically detect and handle specific errors. It
-  is intentionally an enum, because if you need to detect specific errors, the codes need to be a defined list of
-  values - which is an enum.
-- `key`: optionally, a key for the error. This can be used to group errors by keys, for example to group errors by
-   properties that exist on a command message.
+- `key`: optionally, a key for the error. Use this to group errors by keys, for example to group errors by
+  properties that exist on a command message. The key can be a string or enum.
 
 Error objects _must_ be instantiated with either a code or a message. They are immutable, so you cannot change their
 properties after they have been created.
@@ -181,7 +181,7 @@ The error list class is iterable, countable and has `isEmpty()` and `isNotEmpty(
 ### Keyed Error Sets
 
 If you are using the `key` property on error objects, we provide a keyed set of errors class that groups errors by their
-key.
+key. Keys can be strings or enums.
 
 To create one, provide error objects to its constructor:
 
@@ -243,6 +243,16 @@ if ($errors->contains(CancelAttendeeTicketError::AlreadyCancelled)) {
 }
 ```
 
+If you only want to handle one error code at once, use the `code()` method on the errors list. This will return the first error code, or `null` if there are no error codes.
+
+```php
+$explanation = match ($errors->code()) {
+    CancelAttendeeTicketError::AlreadyCancelled => 'The ticket has already been cancelled.',
+    CancelAttendeeTicketError::NotFound => 'The ticket was not found.',
+    default => null,
+};
+```
+
 ## Exception
 
 We provide a `FailedResultException` that you can use to throw a result object:
diff --git a/src/Contracts/Toolkit/Result/Error.php b/src/Contracts/Toolkit/Result/Error.php
index b4455cfe..9b3239e7 100644
--- a/src/Contracts/Toolkit/Result/Error.php
+++ b/src/Contracts/Toolkit/Result/Error.php
@@ -12,16 +12,16 @@
 
 namespace CloudCreativity\Modules\Contracts\Toolkit\Result;
 
-use BackedEnum;
+use UnitEnum;
 
 interface Error
 {
     /**
      * Get the error key.
      *
-     * @return string|null
+     * @return UnitEnum|string|null
      */
-    public function key(): ?string;
+    public function key(): UnitEnum|string|null;
 
     /**
      * Get the error detail.
@@ -33,15 +33,15 @@ public function message(): string;
     /**
      * Get the error code.
      *
-     * @return BackedEnum|null
+     * @return UnitEnum|null
      */
-    public function code(): ?BackedEnum;
+    public function code(): ?UnitEnum;
 
     /**
      * Is the error the specified error code?
      *
-     * @param BackedEnum $code
+     * @param UnitEnum $code
      * @return bool
      */
-    public function is(BackedEnum $code): bool;
+    public function is(UnitEnum $code): bool;
 }
diff --git a/src/Contracts/Toolkit/Result/ListOfErrors.php b/src/Contracts/Toolkit/Result/ListOfErrors.php
index 0044b8a6..26d35f2d 100644
--- a/src/Contracts/Toolkit/Result/ListOfErrors.php
+++ b/src/Contracts/Toolkit/Result/ListOfErrors.php
@@ -12,9 +12,9 @@
 
 namespace CloudCreativity\Modules\Contracts\Toolkit\Result;
 
-use BackedEnum;
 use Closure;
 use CloudCreativity\Modules\Contracts\Toolkit\Iterables\ListIterator;
+use UnitEnum;
 
 /**
  * @extends ListIterator<Error>
@@ -24,26 +24,33 @@ interface ListOfErrors extends ListIterator
     /**
      * Get the first error in the list, or the first matching error.
      *
-     * @param Closure(Error): bool|BackedEnum|null $matcher
+     * @param Closure(Error): bool|UnitEnum|null $matcher
      * @return Error|null
      */
-    public function first(Closure|BackedEnum|null $matcher = null): ?Error;
+    public function first(Closure|UnitEnum|null $matcher = null): ?Error;
 
     /**
      * Does the list contain a matching error?
      *
-     * @param Closure(Error): bool|BackedEnum $matcher
+     * @param Closure(Error): bool|UnitEnum $matcher
      * @return bool
      */
-    public function contains(Closure|BackedEnum $matcher): bool;
+    public function contains(Closure|UnitEnum $matcher): bool;
 
     /**
      * Get all the unique error codes in the list.
      *
-     * @return array<BackedEnum>
+     * @return array<UnitEnum>
      */
     public function codes(): array;
 
+    /**
+     * Get the first error code in the list.
+     *
+     * @return UnitEnum|null
+     */
+    public function code(): ?UnitEnum;
+
     /**
      * Return a new instance with the provided error pushed on to the end of the list.
      *
diff --git a/src/Toolkit/Loggable/ResultDecorator.php b/src/Toolkit/Loggable/ResultDecorator.php
index 1ebda2c7..ea64f0bd 100644
--- a/src/Toolkit/Loggable/ResultDecorator.php
+++ b/src/Toolkit/Loggable/ResultDecorator.php
@@ -17,6 +17,8 @@
 use CloudCreativity\Modules\Contracts\Toolkit\Result\Error;
 use CloudCreativity\Modules\Contracts\Toolkit\Result\Result;
 
+use function CloudCreativity\Modules\Toolkit\enum_string;
+
 final readonly class ResultDecorator implements ContextProvider
 {
     /**
@@ -85,8 +87,10 @@ private function error(Error $error): array
             return $error->context();
         }
 
+        $code = $error->code();
+
         return array_filter([
-            'code' => $error->code()?->value,
+            'code' => $code ? enum_string($code) : null,
             'key' => $error->key(),
             'message' => $error->message(),
         ]);
diff --git a/src/Toolkit/Result/Error.php b/src/Toolkit/Result/Error.php
index 6dcc5fd5..37ad5ac0 100644
--- a/src/Toolkit/Result/Error.php
+++ b/src/Toolkit/Result/Error.php
@@ -12,28 +12,28 @@
 
 namespace CloudCreativity\Modules\Toolkit\Result;
 
-use BackedEnum;
 use CloudCreativity\Modules\Contracts\Toolkit\Result\Error as IError;
 use CloudCreativity\Modules\Toolkit\Contracts;
+use UnitEnum;
 
 final readonly class Error implements IError
 {
     /**
-     * @var string|null
+     * @var UnitEnum|string|null
      */
-    private ?string $key;
+    private UnitEnum|string|null $key;
 
     /**
      * Error constructor.
      *
-     * @param string|null $key
+     * @param UnitEnum|null $code
      * @param string $message
-     * @param BackedEnum|null $code
+     * @param UnitEnum|string|null $key
      */
     public function __construct(
-        ?string $key = null,
+        private ?UnitEnum $code = null,
         private string $message = '',
-        private ?BackedEnum $code = null,
+        UnitEnum|string|null $key = null,
     ) {
         Contracts::assert(!empty($message) || $code !== null, 'Error must have a message or a code.');
         $this->key = $key ?: null;
@@ -42,7 +42,7 @@ public function __construct(
     /**
      * @inheritDoc
      */
-    public function key(): ?string
+    public function key(): UnitEnum|string|null
     {
         return $this->key;
     }
@@ -58,7 +58,7 @@ public function message(): string
     /**
      * @inheritDoc
      */
-    public function code(): ?BackedEnum
+    public function code(): ?UnitEnum
     {
         return $this->code;
     }
@@ -66,7 +66,7 @@ public function code(): ?BackedEnum
     /**
      * @inheritDoc
      */
-    public function is(BackedEnum $code): bool
+    public function is(UnitEnum $code): bool
     {
         return $this->code === $code;
     }
diff --git a/src/Toolkit/Result/KeyedSetOfErrors.php b/src/Toolkit/Result/KeyedSetOfErrors.php
index e8dc2a78..bf77695a 100644
--- a/src/Toolkit/Result/KeyedSetOfErrors.php
+++ b/src/Toolkit/Result/KeyedSetOfErrors.php
@@ -16,6 +16,9 @@
 use CloudCreativity\Modules\Contracts\Toolkit\Result\Error as IError;
 use CloudCreativity\Modules\Contracts\Toolkit\Result\ListOfErrors as IListOfErrors;
 use CloudCreativity\Modules\Toolkit\Iterables\IsKeyedSet;
+use UnitEnum;
+
+use function CloudCreativity\Modules\Toolkit\enum_string;
 
 /**
  * @implements KeyedSet<IListOfErrors>
@@ -55,7 +58,7 @@ public function __construct(IError ...$errors)
             $this->stack[$key] = $this->get($key)->push($error);
         }
 
-        ksort($this->stack);
+        ksort($this->stack, SORT_STRING | SORT_FLAG_CASE);
     }
 
     /**
@@ -105,11 +108,13 @@ public function keys(): array
     /**
      * Get errors by key.
      *
-     * @param string $key
+     * @param UnitEnum|string $key
      * @return IListOfErrors
      */
-    public function get(string $key): IListOfErrors
+    public function get(UnitEnum|string $key): IListOfErrors
     {
+        $key = enum_string($key);
+
         return $this->stack[$key] ?? new ListOfErrors();
     }
 
@@ -151,6 +156,8 @@ public function count(): int
      */
     private function keyFor(IError $error): string
     {
-        return $error->key() ?? self::DEFAULT_KEY;
+        $key = $error->key();
+
+        return $key === null ? self::DEFAULT_KEY : enum_string($key);
     }
 }
diff --git a/src/Toolkit/Result/ListOfErrors.php b/src/Toolkit/Result/ListOfErrors.php
index 64651fba..c4bee207 100644
--- a/src/Toolkit/Result/ListOfErrors.php
+++ b/src/Toolkit/Result/ListOfErrors.php
@@ -12,11 +12,11 @@
 
 namespace CloudCreativity\Modules\Toolkit\Result;
 
-use BackedEnum;
 use Closure;
 use CloudCreativity\Modules\Contracts\Toolkit\Result\Error as IError;
 use CloudCreativity\Modules\Contracts\Toolkit\Result\ListOfErrors as IListOfErrors;
 use CloudCreativity\Modules\Toolkit\Iterables\IsList;
+use UnitEnum;
 
 final class ListOfErrors implements IListOfErrors
 {
@@ -24,17 +24,17 @@ final class ListOfErrors implements IListOfErrors
     use IsList;
 
     /**
-     * @param IListOfErrors|IError|BackedEnum|array<IError>|string $value
+     * @param IListOfErrors|IError|UnitEnum|array<IError>|string $value
      * @return self
      */
-    public static function from(IListOfErrors|IError|BackedEnum|array|string $value): self
+    public static function from(IListOfErrors|IError|UnitEnum|array|string $value): self
     {
         return match(true) {
             $value instanceof self => $value,
             $value instanceof IListOfErrors, is_array($value) => new self(...$value),
             $value instanceof IError => new self($value),
             is_string($value) => new self(new Error(message: $value)),
-            $value instanceof BackedEnum => new self(new Error(code: $value)),
+            $value instanceof UnitEnum => new self(new Error(code: $value)),
         };
     }
 
@@ -49,13 +49,13 @@ public function __construct(IError ...$errors)
     /**
      * @inheritDoc
      */
-    public function first(Closure|BackedEnum|null $matcher = null): ?IError
+    public function first(Closure|UnitEnum|null $matcher = null): ?IError
     {
         if ($matcher === null) {
             return $this->stack[0] ?? null;
         }
 
-        if ($matcher instanceof BackedEnum) {
+        if ($matcher instanceof UnitEnum) {
             $matcher = static fn (IError $error): bool => $error->is($matcher);
         }
 
@@ -71,9 +71,9 @@ public function first(Closure|BackedEnum|null $matcher = null): ?IError
     /**
      * @inheritDoc
      */
-    public function contains(Closure|BackedEnum $matcher): bool
+    public function contains(Closure|UnitEnum $matcher): bool
     {
-        if ($matcher instanceof BackedEnum) {
+        if ($matcher instanceof UnitEnum) {
             $matcher = static fn (IError $error): bool => $error->is($matcher);
         }
 
@@ -104,6 +104,20 @@ public function codes(): array
         return $codes;
     }
 
+    /**
+     * @inheritDoc
+     */
+    public function code(): ?UnitEnum
+    {
+        foreach ($this->stack as $error) {
+            if ($code = $error->code()) {
+                return $code;
+            }
+        }
+
+        return null;
+    }
+
     /**
      * @inheritDoc
      */
diff --git a/src/Toolkit/Result/Result.php b/src/Toolkit/Result/Result.php
index 13236701..b8e45117 100644
--- a/src/Toolkit/Result/Result.php
+++ b/src/Toolkit/Result/Result.php
@@ -12,10 +12,10 @@
 
 namespace CloudCreativity\Modules\Toolkit\Result;
 
-use BackedEnum;
 use CloudCreativity\Modules\Contracts\Toolkit\Result\Error as IError;
 use CloudCreativity\Modules\Contracts\Toolkit\Result\ListOfErrors as IListOfErrors;
 use CloudCreativity\Modules\Contracts\Toolkit\Result\Result as IResult;
+use UnitEnum;
 
 /**
  * @template TValue
@@ -38,10 +38,10 @@ public static function ok(mixed $value = null): self
     /**
      * Return a failed result.
      *
-     * @param IListOfErrors|IError|BackedEnum|array<IError>|string $errorOrErrors
+     * @param IListOfErrors|IError|UnitEnum|array<IError>|string $errorOrErrors
      * @return Result<null>
      */
-    public static function failed(IListOfErrors|IError|BackedEnum|array|string $errorOrErrors): self
+    public static function failed(IListOfErrors|IError|UnitEnum|array|string $errorOrErrors): self
     {
         $errors = match(true) {
             $errorOrErrors instanceof IListOfErrors => $errorOrErrors,
@@ -58,10 +58,10 @@ public static function failed(IListOfErrors|IError|BackedEnum|array|string $erro
      *
      * This is an alias for the `failed` method.
      *
-     * @param IListOfErrors|IError|BackedEnum|array<IError>|string $errorOrErrors
+     * @param IListOfErrors|IError|UnitEnum|array<IError>|string $errorOrErrors
      * @return Result<null>
      */
-    public static function fail(IListOfErrors|IError|BackedEnum|array|string $errorOrErrors): self
+    public static function fail(IListOfErrors|IError|UnitEnum|array|string $errorOrErrors): self
     {
         return self::failed($errorOrErrors);
     }
diff --git a/tests/Unit/Toolkit/Loggable/ResultDecoratorTest.php b/tests/Unit/Toolkit/Loggable/ResultDecoratorTest.php
index 3ba55f74..d6dfc2bb 100644
--- a/tests/Unit/Toolkit/Loggable/ResultDecoratorTest.php
+++ b/tests/Unit/Toolkit/Loggable/ResultDecoratorTest.php
@@ -18,12 +18,16 @@
 use CloudCreativity\Modules\Contracts\Toolkit\Result\Error as IError;
 use CloudCreativity\Modules\Contracts\Toolkit\Result\Result as IResult;
 use CloudCreativity\Modules\Tests\TestBackedEnum;
+use CloudCreativity\Modules\Tests\TestUnitEnum;
 use CloudCreativity\Modules\Toolkit\Loggable\ResultDecorator;
 use CloudCreativity\Modules\Toolkit\Loggable\SimpleContextFactory;
 use CloudCreativity\Modules\Toolkit\Result\Error;
 use CloudCreativity\Modules\Toolkit\Result\Result;
 use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
+use UnitEnum;
+
+use function CloudCreativity\Modules\Toolkit\enum_string;
 
 /**
  * @extends IResult<null>
@@ -200,12 +204,12 @@ public function testFailureContextWithErrorThatOnlyHasMessage(string|Error $erro
     }
 
     /**
-     * @return array<array<BackedEnum|Error>>
+     * @return array<array<UnitEnum|Error>>
      */
     public static function onlyCodeProvider(): array
     {
         return [
-            [TestBackedEnum::Foo],
+            [TestUnitEnum::Baz],
             [new Error(code: TestBackedEnum::Bar)],
         ];
     }
@@ -215,13 +219,14 @@ public static function onlyCodeProvider(): array
      * @return void
      */
     #[DataProvider('onlyCodeProvider')]
-    public function testFailureContextWithErrorThatOnlyHasCode(BackedEnum|Error $error): void
+    public function testFailureContextWithErrorThatOnlyHasCode(UnitEnum|Error $error): void
     {
         $result = Result::failed($error);
+        $code = $error instanceof UnitEnum ? $error : $error->code();
 
         $expected = [
             'success' => false,
-            'error' => $error instanceof BackedEnum ? $error->value : $error->code()?->value,
+            'error' => enum_string($code ?? '!!'),
         ];
 
         $this->assertSame($expected, (new ResultDecorator($result))->context());
diff --git a/tests/Unit/Toolkit/Result/ErrorTest.php b/tests/Unit/Toolkit/Result/ErrorTest.php
index cda7b69a..046992f3 100644
--- a/tests/Unit/Toolkit/Result/ErrorTest.php
+++ b/tests/Unit/Toolkit/Result/ErrorTest.php
@@ -14,30 +14,25 @@
 
 use CloudCreativity\Modules\Contracts\Toolkit\Result\Error as IError;
 use CloudCreativity\Modules\Tests\TestBackedEnum;
+use CloudCreativity\Modules\Tests\TestUnitEnum;
 use CloudCreativity\Modules\Toolkit\ContractException;
 use CloudCreativity\Modules\Toolkit\Result\Error;
 use PHPUnit\Framework\TestCase;
 
 class ErrorTest extends TestCase
 {
-    /**
-     * @return void
-     */
     public function test(): void
     {
-        $error = new Error(key: 'foo', message: 'Bar', code: TestBackedEnum::Foo);
+        $error = new Error(key: 'foo', message: 'Bar', code: TestUnitEnum::Baz);
 
         $this->assertInstanceOf(IError::class, $error);
         $this->assertSame('foo', $error->key());
         $this->assertSame('Bar', $error->message());
-        $this->assertSame(TestBackedEnum::Foo, $error->code());
-        $this->assertTrue($error->is(TestBackedEnum::Foo));
-        $this->assertFalse($error->is(TestBackedEnum::Bar));
+        $this->assertSame(TestUnitEnum::Baz, $error->code());
+        $this->assertTrue($error->is(TestUnitEnum::Baz));
+        $this->assertFalse($error->is(TestUnitEnum::Bat));
     }
 
-    /**
-     * @return void
-     */
     public function testOnlyMessage(): void
     {
         $error = new Error(message: 'Hello World');
@@ -47,9 +42,6 @@ public function testOnlyMessage(): void
         $this->assertNull($error->code());
     }
 
-    /**
-     * @return void
-     */
     public function testOnlyCode(): void
     {
         $error = new Error(code: TestBackedEnum::Foo);
@@ -59,9 +51,6 @@ public function testOnlyCode(): void
         $this->assertSame(TestBackedEnum::Foo, $error->code());
     }
 
-    /**
-     * @return void
-     */
     public function testNoMessageOrCode(): void
     {
         $this->expectException(ContractException::class);
diff --git a/tests/Unit/Toolkit/Result/KeyedSetOfErrorsTest.php b/tests/Unit/Toolkit/Result/KeyedSetOfErrorsTest.php
index 3ca56322..0d5f8337 100644
--- a/tests/Unit/Toolkit/Result/KeyedSetOfErrorsTest.php
+++ b/tests/Unit/Toolkit/Result/KeyedSetOfErrorsTest.php
@@ -12,6 +12,7 @@
 
 namespace CloudCreativity\Modules\Tests\Unit\Toolkit\Result;
 
+use CloudCreativity\Modules\Tests\TestUnitEnum;
 use CloudCreativity\Modules\Toolkit\Result\Error;
 use CloudCreativity\Modules\Toolkit\Result\KeyedSetOfErrors;
 use CloudCreativity\Modules\Toolkit\Result\ListOfErrors;
@@ -25,26 +26,33 @@ class KeyedSetOfErrorsTest extends TestCase
     public function test(): void
     {
         $errors = new KeyedSetOfErrors(
-            $a = new Error('foo', 'Message A'),
-            $b = new Error('bar', 'Message B'),
-            $c = new Error('foo', 'Message C'),
-            $d = new Error(null, 'Message D'),
-            $e = new Error(null, 'Message E'),
+            $a = new Error(message: 'Message A', key: 'foo'),
+            $b = new Error(message: 'Message B', key: 'bar'),
+            $c = new Error(message: 'Message C', key: 'foo'),
+            $d = new Error(message: 'Message D'),
+            $e = new Error(message: 'Message E'),
+            $f = new Error(message: 'Message F', key: TestUnitEnum::Baz),
+            $g = new Error(message: 'Message G', key: TestUnitEnum::Bat),
+            $h = new Error(message: 'Message H', key: TestUnitEnum::Baz),
         );
 
         $expected = [
             '_base' => new ListOfErrors($d, $e),
             'bar' => new ListOfErrors($b),
+            TestUnitEnum::Bat->name => new ListOfErrors($g),
+            TestUnitEnum::Baz->name => new ListOfErrors($f, $h),
             'foo' => new ListOfErrors($a, $c),
         ];
 
         $this->assertEquals($expected, iterator_to_array($errors));
         $this->assertEquals($expected, $errors->all());
-        $this->assertSame(['_base', 'bar', 'foo'], $errors->keys());
-        $this->assertEquals(new ListOfErrors($d, $e, $b, $a, $c), $errors->toList());
-        $this->assertCount(5, $errors);
+        $this->assertSame(['_base', 'bar', TestUnitEnum::Bat->name, TestUnitEnum::Baz->name, 'foo'], $errors->keys());
+        $this->assertEquals(new ListOfErrors($d, $e, $b, $g, $f, $h, $a, $c), $errors->toList());
+        $this->assertCount(8, $errors);
         $this->assertTrue($errors->isNotEmpty());
         $this->assertFalse($errors->isEmpty());
+        $this->assertEquals($expected['bar'], $errors->get('bar'));
+        $this->assertEquals($expected[TestUnitEnum::Baz->name], $errors->get(TestUnitEnum::Baz));
     }
 
     /**
@@ -65,12 +73,12 @@ public function testEmpty(): void
     public function testPutNewKey(): void
     {
         $original = new KeyedSetOfErrors(
-            $a = new Error('foo', 'Message A'),
-            $b = new Error('bar', 'Message B'),
-            $c = new Error('foo', 'Message C'),
+            $a = new Error(message: 'Message A', key: 'foo'),
+            $b = new Error(message: 'Message B', key: 'bar'),
+            $c = new Error(message: 'Message C', key: 'foo'),
         );
 
-        $actual = $original->put($d = new Error('baz', 'Message D'));
+        $actual = $original->put($d = new Error(message: 'Message D', key: 'baz'));
 
         $this->assertNotSame($original, $actual);
         $this->assertEquals([
@@ -91,12 +99,12 @@ public function testPutNewKey(): void
     public function testPutExistingKey(): void
     {
         $original = new KeyedSetOfErrors(
-            $a = new Error('foo', 'Message A'),
-            $b = new Error('bar', 'Message B'),
-            $c = new Error('foo', 'Message C'),
+            $a = new Error(message: 'Message A', key: 'foo'),
+            $b = new Error(message: 'Message B', key: 'bar'),
+            $c = new Error(message: 'Message C', key: 'foo'),
         );
 
-        $actual = $original->put($d = new Error('bar', 'Message D'));
+        $actual = $original->put($d = new Error(message: 'Message D', key: 'bar'));
 
         $this->assertNotSame($original, $actual);
         $this->assertEquals([
@@ -116,12 +124,12 @@ public function testPutExistingKey(): void
     public function testPutErrorWithoutKey1(): void
     {
         $original = new KeyedSetOfErrors(
-            $a = new Error('foo', 'Message A'),
-            $b = new Error('bar', 'Message B'),
-            $c = new Error('foo', 'Message C'),
+            $a = new Error(message: 'Message A', key: 'foo'),
+            $b = new Error(message: 'Message B', key: 'bar'),
+            $c = new Error(message: 'Message C', key: 'foo'),
         );
 
-        $actual = $original->put($d = new Error(null, 'Message D'));
+        $actual = $original->put($d = new Error(message: 'Message D'));
 
         $this->assertNotSame($original, $actual);
         $this->assertEquals([
@@ -142,12 +150,12 @@ public function testPutErrorWithoutKey1(): void
     public function testPutErrorWithoutKey2(): void
     {
         $original = new KeyedSetOfErrors(
-            $a = new Error(null, 'Message A'),
-            $b = new Error('foo', 'Message B'),
-            $c = new Error(null, 'Message C'),
+            $a = new Error(message: 'Message A'),
+            $b = new Error(message: 'Message B', key: 'foo'),
+            $c = new Error(message: 'Message C'),
         );
 
-        $actual = $original->put($d = new Error(null, 'Message D'));
+        $actual = $original->put($d = new Error(message: 'Message D'));
 
         $this->assertNotSame($original, $actual);
         $this->assertEquals([
@@ -167,16 +175,16 @@ public function testPutErrorWithoutKey2(): void
     public function testMerge(): void
     {
         $set1 = new KeyedSetOfErrors(
-            $a = new Error('foo', 'Message A'),
-            $b = new Error('bar', 'Message B'),
-            $c = new Error('foo', 'Message C'),
+            $a = new Error(message: 'Message A', key: 'foo'),
+            $b = new Error(message: 'Message B', key: 'bar'),
+            $c = new Error(message: 'Message C', key: 'foo'),
         );
 
         $set2 = new KeyedSetOfErrors(
-            $d = new Error('bar', 'Message D'),
-            $e = new Error('baz', 'Message E'),
-            $f = new Error('bar', 'Message F'),
-            $g = new Error(null, 'Message G'),
+            $d = new Error(message: 'Message D', key: 'bar'),
+            $e = new Error(message: 'Message E', key: 'baz'),
+            $f = new Error(message: 'Message F', key: 'bar'),
+            $g = new Error(message: 'Message G'),
         );
 
         $actual = $set1->merge($set2);
diff --git a/tests/Unit/Toolkit/Result/ListOfErrorsTest.php b/tests/Unit/Toolkit/Result/ListOfErrorsTest.php
index e405ea4a..b5da8588 100644
--- a/tests/Unit/Toolkit/Result/ListOfErrorsTest.php
+++ b/tests/Unit/Toolkit/Result/ListOfErrorsTest.php
@@ -15,6 +15,7 @@
 use CloudCreativity\Modules\Contracts\Toolkit\Result\Error as IError;
 use CloudCreativity\Modules\Contracts\Toolkit\Result\ListOfErrors as IListOfErrors;
 use CloudCreativity\Modules\Tests\TestBackedEnum;
+use CloudCreativity\Modules\Tests\TestUnitEnum;
 use CloudCreativity\Modules\Toolkit\Result\Error;
 use CloudCreativity\Modules\Toolkit\Result\KeyedSetOfErrors;
 use CloudCreativity\Modules\Toolkit\Result\ListOfErrors;
@@ -106,14 +107,14 @@ public function testFirst(): void
             new Error(null, 'Message B'),
             $c = new Error(null, 'Message C'),
             new Error(null, 'Message D'),
-            $e = new Error(code: TestBackedEnum::Bar),
+            $e = new Error(code: TestUnitEnum::Bat),
         );
 
         $this->assertSame($a, $errors->first());
         $this->assertSame($c, $errors->first(fn (IError $error) => 'Message C' === $error->message()));
-        $this->assertSame($e, $errors->first(TestBackedEnum::Bar));
+        $this->assertSame($e, $errors->first(TestUnitEnum::Bat));
         $this->assertNull($errors->first(fn (IError $error) => 'Message E' === $error->message()));
-        $this->assertNull($errors->first(TestBackedEnum::Foo));
+        $this->assertNull($errors->first(TestUnitEnum::Baz));
     }
 
     /**
@@ -125,27 +126,48 @@ public function testContains(): void
             new Error(message: 'Message A'),
             new Error(message: 'Message B'),
             new Error(message: 'Message C'),
-            new Error(message: 'Message D', code: TestBackedEnum::Foo),
+            new Error(message: 'Message D', code: TestUnitEnum::Baz),
         );
 
         $this->assertTrue($errors->contains(fn (IError $error) => 'Message C' === $error->message()));
-        $this->assertTrue($errors->contains(TestBackedEnum::Foo));
+        $this->assertTrue($errors->contains(TestUnitEnum::Baz));
         $this->assertFalse($errors->contains(fn (IError $error) => 'Message E' === $error->message()));
-        $this->assertFalse($errors->contains(TestBackedEnum::Bar));
+        $this->assertFalse($errors->contains(TestUnitEnum::Bat));
     }
 
-    /**
-     * @return void
-     */
     public function testCodes(): void
     {
-        $errors = new ListOfErrors(
+        $errors1 = new ListOfErrors(
             new Error(message: 'Message A'),
             new Error(message: 'Message B', code: TestBackedEnum::Foo),
-            new Error(message: 'Message C', code: TestBackedEnum::Bar),
+            new Error(message: 'Message C', code: TestUnitEnum::Baz),
             new Error(message: 'Message D', code: TestBackedEnum::Foo),
         );
 
-        $this->assertSame([TestBackedEnum::Foo, TestBackedEnum::Bar], $errors->codes());
+        $errors2 = new ListOfErrors(
+            new Error(message: 'Message E'),
+            new Error(message: 'Message F'),
+        );
+
+        $this->assertSame([TestBackedEnum::Foo, TestUnitEnum::Baz], $errors1->codes());
+        $this->assertEmpty($errors2->codes());
+    }
+
+    public function testCode(): void
+    {
+        $errors1 = new ListOfErrors(
+            new Error(message: 'Message A'),
+            new Error(message: 'Message B', code: TestBackedEnum::Foo),
+            new Error(message: 'Message C', code: TestUnitEnum::Baz),
+            new Error(message: 'Message D', code: TestBackedEnum::Foo),
+        );
+
+        $errors2 = new ListOfErrors(
+            new Error(message: 'Message E'),
+            new Error(message: 'Message F'),
+        );
+
+        $this->assertSame(TestBackedEnum::Foo, $errors1->code());
+        $this->assertNull($errors2->code());
     }
 }
diff --git a/tests/Unit/Toolkit/Result/ResultTest.php b/tests/Unit/Toolkit/Result/ResultTest.php
index c17bf57f..95e27e74 100644
--- a/tests/Unit/Toolkit/Result/ResultTest.php
+++ b/tests/Unit/Toolkit/Result/ResultTest.php
@@ -15,6 +15,7 @@
 use CloudCreativity\Modules\Contracts\Toolkit\Result\ListOfErrors as IListOfErrors;
 use CloudCreativity\Modules\Contracts\Toolkit\Result\Result as IResult;
 use CloudCreativity\Modules\Tests\TestBackedEnum;
+use CloudCreativity\Modules\Tests\TestUnitEnum;
 use CloudCreativity\Modules\Toolkit\Result\Error;
 use CloudCreativity\Modules\Toolkit\Result\FailedResultException;
 use CloudCreativity\Modules\Toolkit\Result\ListOfErrors;
@@ -100,7 +101,7 @@ public function testErrorWithMultipleErrors(): void
     {
         $errors = new ListOfErrors(
             new Error(code: TestBackedEnum::Foo),
-            new Error(code: TestBackedEnum::Bar),
+            new Error(code: TestUnitEnum::Baz),
             new Error(message: 'Message A'),
             new Error(message: 'Message B'),
         );