diff --git a/src/Injector.php b/src/Injector.php
index 640fdcb..3d67da5 100644
--- a/src/Injector.php
+++ b/src/Injector.php
@@ -4,6 +4,7 @@
 use ReflectionClass;
 use ReflectionFunction;
 use ReflectionNamedType;
+use ReflectionParameter;
 
 class Injector {
 	public function __construct(
@@ -36,28 +37,7 @@ public function invoke(
 		}
 
 		foreach($refFunction->getParameters() as $refParam) {
-			/** @var ReflectionNamedType|null $refType */
-			$refType = $refParam->getType();
-			/** @var class-string $refParamTypeName */
-			$refParamTypeName = $refType->getName();
-
-			try {
-				array_push(
-					$arguments,
-					$extraArgs[$refParamTypeName] ?? $this->container->get($refParamTypeName)
-				);
-			}
-			catch(ServiceNotFoundException $exception) {
-				if($refType->allowsNull()) {
-					array_push(
-						$arguments,
-						null,
-					);
-				}
-				else {
-					throw $exception;
-				}
-			}
+			array_push($arguments, $this->resolveParameter($refParam, $extraArgs));
 		}
 
 		if($instance) {
@@ -66,4 +46,77 @@ public function invoke(
 
 		return $refFunction->invoke(...$arguments);
 	}
+
+	/** @param array<string, mixed> $extraArgs */
+	private function resolveParameter(
+		ReflectionParameter $refParam,
+		array $extraArgs,
+	):mixed {
+		$refType = $refParam->getType();
+		if(!$refType instanceof ReflectionNamedType) {
+			return $this->resolveUntypedParameter($refParam, $extraArgs);
+		}
+
+		/** @var class-string $refParamTypeName */
+		$refParamTypeName = $refType->getName();
+
+		if(array_key_exists($refParam->getName(), $extraArgs)) {
+			return $extraArgs[$refParam->getName()];
+		}
+		if(array_key_exists($refParamTypeName, $extraArgs)) {
+			return $extraArgs[$refParamTypeName];
+		}
+		if($refParam->isDefaultValueAvailable()) {
+			return $refParam->getDefaultValue();
+		}
+		if($refType->isBuiltin()) {
+			return $this->resolveBuiltinParameter($refType, $refParamTypeName);
+		}
+
+		return $this->resolveServiceParameter($refType, $refParamTypeName);
+	}
+
+	/** @param array<string, mixed> $extraArgs */
+	private function resolveUntypedParameter(
+		ReflectionParameter $refParam,
+		array $extraArgs,
+	):mixed {
+		if(array_key_exists($refParam->getName(), $extraArgs)) {
+			return $extraArgs[$refParam->getName()];
+		}
+		if($refParam->isDefaultValueAvailable()) {
+			return $refParam->getDefaultValue();
+		}
+
+		return null;
+	}
+
+	/** @param class-string $refParamTypeName */
+	private function resolveBuiltinParameter(
+		ReflectionNamedType $refType,
+		string $refParamTypeName,
+	):mixed {
+		if($refType->allowsNull()) {
+			return null;
+		}
+
+		throw new ServiceNotFoundException($refParamTypeName);
+	}
+
+	/** @param class-string $refParamTypeName */
+	private function resolveServiceParameter(
+		ReflectionNamedType $refType,
+		string $refParamTypeName,
+	):mixed {
+		try {
+			return $this->container->get($refParamTypeName);
+		}
+		catch(ServiceNotFoundException $exception) {
+			if($refType->allowsNull()) {
+				return null;
+			}
+
+			throw $exception;
+		}
+	}
 }
diff --git a/test/phpunit/InjectorTest.php b/test/phpunit/InjectorTest.php
index ac62b0d..5da9afa 100644
--- a/test/phpunit/InjectorTest.php
+++ b/test/phpunit/InjectorTest.php
@@ -4,6 +4,7 @@
 use DateTime;
 use GT\ServiceContainer\Container;
 use GT\ServiceContainer\Injector;
+use GT\ServiceContainer\ServiceNotFoundException;
 use GT\ServiceContainer\Test\Example\Greeter;
 use PHPUnit\Framework\TestCase;
 
@@ -56,4 +57,108 @@ public function testInvoke_noClass():void {
 		self::assertCount(1, $invocationList);
 		self::assertSame($greeter, $invocationList[0]);
 	}
+
+	public function testInvoke_usesDefaultScalarParameter():void {
+		$function = function(string $name = "World"):string {
+			return "Hello, $name!";
+		};
+
+		$sut = new Injector(new Container());
+		self::assertSame("Hello, World!", $sut->invoke(null, $function));
+	}
+
+	public function testInvoke_usesExtraArgByParameterName():void {
+		$function = function(string $name = "World"):string {
+			return "Hello, $name!";
+		};
+
+		$sut = new Injector(new Container());
+		self::assertSame(
+			"Hello, Cron!",
+			$sut->invoke(null, $function, ["name" => "Cron"])
+		);
+	}
+
+	public function testInvoke_usesExtraArgByParameterType():void {
+		$dateTime = new DateTime("2026-06-22");
+		$function = function(DateTime $dateTime):string {
+			return $dateTime->format("Y-m-d");
+		};
+
+		$sut = new Injector(new Container());
+		self::assertSame(
+			"2026-06-22",
+			$sut->invoke(null, $function, [DateTime::class => $dateTime])
+		);
+	}
+
+	public function testInvoke_usesExtraArgForUntypedParameter():void {
+		$function = function($name = "World"):string {
+			return "Hello, $name!";
+		};
+
+		$sut = new Injector(new Container());
+		self::assertSame(
+			"Hello, Cron!",
+			$sut->invoke(null, $function, ["name" => "Cron"])
+		);
+	}
+
+	public function testInvoke_usesDefaultUntypedParameter():void {
+		$function = function($name = "World"):string {
+			return "Hello, $name!";
+		};
+
+		$sut = new Injector(new Container());
+		self::assertSame("Hello, World!", $sut->invoke(null, $function));
+	}
+
+	public function testInvoke_setsMissingUntypedParameterToNull():void {
+		$function = function($name):mixed {
+			return $name;
+		};
+
+		$sut = new Injector(new Container());
+		self::assertNull($sut->invoke(null, $function));
+	}
+
+	public function testInvoke_setsMissingNullableBuiltinParameterToNull():void {
+		$function = function(?string $name):mixed {
+			return $name;
+		};
+
+		$sut = new Injector(new Container());
+		self::assertNull($sut->invoke(null, $function));
+	}
+
+	public function testInvoke_throwsWhenRequiredBuiltinParameterIsMissing():void {
+		$function = function(string $name):string {
+			return $name;
+		};
+
+		$sut = new Injector(new Container());
+
+		self::expectException(ServiceNotFoundException::class);
+		$sut->invoke(null, $function);
+	}
+
+	public function testInvoke_setsMissingNullableServiceParameterToNull():void {
+		$function = function(?Greeter $greeter):mixed {
+			return $greeter;
+		};
+
+		$sut = new Injector(new Container());
+		self::assertNull($sut->invoke(null, $function));
+	}
+
+	public function testInvoke_throwsWhenRequiredServiceParameterIsMissing():void {
+		$function = function(Greeter $greeter):Greeter {
+			return $greeter;
+		};
+
+		$sut = new Injector(new Container());
+
+		self::expectException(ServiceNotFoundException::class);
+		$sut->invoke(null, $function);
+	}
 }