diff --git a/src/DeepClone/DeepClone.php b/src/DeepClone/DeepClone.php
index 2711034a..197ee3b6 100644
--- a/src/DeepClone/DeepClone.php
+++ b/src/DeepClone/DeepClone.php
@@ -398,12 +398,16 @@ public static function deepclone_hydrate(object|string $object_or_class, array $
 
         if (\is_string($class = $object_or_class)) {
             $r = self::$reflectors[$class] ??= self::getClassReflector($class);
-            if (self::$cloneable[$class]) {
+            if (null === self::$prototypes[$class] && !self::$instantiableWithoutConstructor[$class]) {
+                // No empty-shell prototype exists (e.g. an internal final class
+                // whose __unserialize() rejects an empty payload, like
+                // BcMath\Number). Such a class can only be reconstructed via a
+                // full serialization round-trip, never by property injection.
+                throw new \DeepClone\NotInstantiableException('Class "'.$class.'" is not instantiable.');
+            } elseif (self::$cloneable[$class]) {
                 $object = clone self::$prototypes[$class];
             } elseif (self::$instantiableWithoutConstructor[$class]) {
                 $object = $r->newInstanceWithoutConstructor();
-            } elseif (null === self::$prototypes[$class]) {
-                throw new \DeepClone\NotInstantiableException('Class "'.$class.'" is not instantiable.');
             } elseif ($r->implementsInterface('Serializable') && !method_exists($class, '__unserialize')) {
                 $object = unserialize('C:'.\strlen($class).':"'.$class.'":0:{}');
             } else {
@@ -924,6 +928,14 @@ private static function reconstruct($prepared, $objectMeta, $numObjects, $proper
                     continue;
                 }
                 $objClass = $obj::class;
+                if (self::$needsFullUnserialize[$objectMeta[$zid][0]] ?? false) {
+                    // Already fully reconstructed via the full O: serialization
+                    // form (eager-finalize loop above), which invokes
+                    // __unserialize() internally. Calling it again would re-init
+                    // an already-initialized (often readonly) object, e.g.
+                    // BcMath\Number throws "Cannot modify readonly property".
+                    continue;
+                }
                 if (!method_exists($obj, '__unserialize')) {
                     throw new \ValueError('deepclone_from_array(): Argument #1 ($data) "states" entry references object id '.$zid.' whose class '.$objClass.' has no __unserialize() method');
                 }
diff --git a/tests/DeepClone/DeepCloneTest.php b/tests/DeepClone/DeepCloneTest.php
index 78fee2c5..6d152894 100644
--- a/tests/DeepClone/DeepCloneTest.php
+++ b/tests/DeepClone/DeepCloneTest.php
@@ -1903,4 +1903,57 @@ public function testHydrateEnumCastAppliesEvenWhenHookParamIsWider()
         $this->assertSame(DeepCloneHydrateSuit::Spades, $o->s);
         $this->assertNull(HookedEnumWiderParam::$lastRaw);
     }
+
+    /**
+     * @requires PHP 8.4
+     * @requires extension bcmath
+     */
+    public function testRoundTripBcMathNumber()
+    {
+        // BcMath\Number is a final internal class with a custom create_object,
+        // so newInstanceWithoutConstructor() is rejected and an empty O:
+        // unserialize is refused by __unserialize(). The round-trip must still
+        // reconstruct it through a full serialization replay.
+        $n = new \BcMath\Number('12.34');
+        $o = (object) ['a' => $n, 'b' => $n, 'list' => [$n, new \BcMath\Number('5')]];
+
+        $c = deepclone_from_array(deepclone_to_array($o));
+
+        $this->assertInstanceOf(\BcMath\Number::class, $c->a);
+        $this->assertSame('12.34', (string) $c->a);
+        $this->assertSame(2, $c->a->scale);
+        $this->assertNotSame($n, $c->a, 'is a real clone, not the original instance');
+        $this->assertSame($c->a, $c->b, 'shared identity is preserved');
+        $this->assertSame($c->a, $c->list[0], 'shared identity is preserved across the graph');
+        $this->assertSame('5', (string) $c->list[1]);
+    }
+
+    /**
+     * @requires PHP 8.4
+     * @requires extension bcmath
+     */
+    public function testRoundTripBcMathNumberTopLevel()
+    {
+        $c = deepclone_from_array(deepclone_to_array(new \BcMath\Number('99.999')));
+
+        $this->assertInstanceOf(\BcMath\Number::class, $c);
+        $this->assertSame('99.999', (string) $c);
+        $this->assertSame(3, $c->scale);
+        $this->assertSame('100.499', (string) $c->add('0.5'));
+    }
+
+    /**
+     * @requires PHP 8.4
+     * @requires extension bcmath
+     */
+    public function testHydrateBcMathNumberThrows()
+    {
+        // deepclone_hydrate() injects properties into an empty shell; a class
+        // that only becomes valid through __construct()/__unserialize() cannot
+        // be built that way and must be rejected rather than yielding a broken
+        // (uninitialized) instance.
+        $this->expectException(\DeepClone\NotInstantiableException::class);
+        $this->expectExceptionMessage('Class "BcMath\Number" is not instantiable.');
+        deepclone_hydrate(\BcMath\Number::class, ['value' => '7.5']);
+    }
 }