diff --git a/src/Serializer/AbstractItemNormalizer.php b/src/Serializer/AbstractItemNormalizer.php
index b74a97a78a..3b4f395315 100644
--- a/src/Serializer/AbstractItemNormalizer.php
+++ b/src/Serializer/AbstractItemNormalizer.php
@@ -514,16 +514,27 @@ protected function denormalizeCollection(string $attribute, ApiProperty $propert
             throw NotNormalizableValueException::createForUnexpectedDataType(sprintf('The type of the "%s" attribute must be "array", "%s" given.', $attribute, \gettype($value)), $value, [Type::BUILTIN_TYPE_ARRAY], $context['deserialization_path'] ?? null);
         }
 
-        $collectionKeyType = $type->getCollectionKeyTypes()[0] ?? null;
-        $collectionKeyBuiltinType = $collectionKeyType?->getBuiltinType();
-        $childContext = $this->createChildContext($this->createOperationContext($context, $className), $attribute, $format);
         $values = [];
+        $childContext = $this->createChildContext($this->createOperationContext($context, $className), $attribute, $format);
+        $collectionKeyTypes = $type->getCollectionKeyTypes();
         foreach ($value as $index => $obj) {
-            if (null !== $collectionKeyBuiltinType && !\call_user_func('is_'.$collectionKeyBuiltinType, $index)) {
-                throw NotNormalizableValueException::createForUnexpectedDataType(sprintf('The type of the key "%s" must be "%s", "%s" given.', $index, $collectionKeyBuiltinType, \gettype($index)), $index, [$collectionKeyBuiltinType], ($context['deserialization_path'] ?? false) ? sprintf('key(%s)', $context['deserialization_path']) : null, true);
+            // no typehint provided on collection key
+            if (!$collectionKeyTypes) {
+                $values[$index] = $this->denormalizeRelation($attribute, $propertyMetadata, $className, $obj, $format, $childContext);
+                continue;
             }
 
-            $values[$index] = $this->denormalizeRelation($attribute, $propertyMetadata, $className, $obj, $format, $childContext);
+            // validate collection key typehint
+            foreach ($collectionKeyTypes as $collectionKeyType) {
+                $collectionKeyBuiltinType = $collectionKeyType->getBuiltinType();
+                if (!\call_user_func('is_'.$collectionKeyBuiltinType, $index)) {
+                    continue;
+                }
+
+                $values[$index] = $this->denormalizeRelation($attribute, $propertyMetadata, $className, $obj, $format, $childContext);
+                continue 2;
+            }
+            throw NotNormalizableValueException::createForUnexpectedDataType(sprintf('The type of the key "%s" must be "%s", "%s" given.', $index, $collectionKeyTypes[0]->getBuiltinType(), \gettype($index)), $index, [$collectionKeyTypes[0]->getBuiltinType()], ($context['deserialization_path'] ?? false) ? sprintf('key(%s)', $context['deserialization_path']) : null, true);
         }
 
         return $values;
diff --git a/src/Serializer/Tests/AbstractItemNormalizerTest.php b/src/Serializer/Tests/AbstractItemNormalizerTest.php
index 7efd665e02..58a3a7a88a 100644
--- a/src/Serializer/Tests/AbstractItemNormalizerTest.php
+++ b/src/Serializer/Tests/AbstractItemNormalizerTest.php
@@ -878,21 +878,27 @@ public function testDenormalizeWritableLinks(): void
             'name' => 'foo',
             'relatedDummy' => ['foo' => 'bar'],
             'relatedDummies' => [['bar' => 'baz']],
+            'relatedDummiesWithUnionTypes' => [0 => ['bar' => 'qux'], 1. => ['bar' => 'quux']],
         ];
 
         $relatedDummy1 = new RelatedDummy();
         $relatedDummy2 = new RelatedDummy();
+        $relatedDummy3 = new RelatedDummy();
+        $relatedDummy4 = new RelatedDummy();
 
         $propertyNameCollectionFactoryProphecy = $this->prophesize(PropertyNameCollectionFactoryInterface::class);
-        $propertyNameCollectionFactoryProphecy->create(Dummy::class, [])->willReturn(new PropertyNameCollection(['name', 'relatedDummy', 'relatedDummies']));
+        $propertyNameCollectionFactoryProphecy->create(Dummy::class, [])->willReturn(new PropertyNameCollection(['name', 'relatedDummy', 'relatedDummies', 'relatedDummiesWithUnionTypes']));
 
         $relatedDummyType = new Type(Type::BUILTIN_TYPE_OBJECT, false, RelatedDummy::class);
         $relatedDummiesType = new Type(Type::BUILTIN_TYPE_OBJECT, false, ArrayCollection::class, true, new Type(Type::BUILTIN_TYPE_INT), $relatedDummyType);
+        $relatedDummiesWithUnionTypesIntType = new Type(Type::BUILTIN_TYPE_OBJECT, false, ArrayCollection::class, true, new Type(Type::BUILTIN_TYPE_INT), $relatedDummyType);
+        $relatedDummiesWithUnionTypesFloatType = new Type(Type::BUILTIN_TYPE_OBJECT, false, ArrayCollection::class, true, new Type(Type::BUILTIN_TYPE_FLOAT), $relatedDummyType);
 
         $propertyMetadataFactoryProphecy = $this->prophesize(PropertyMetadataFactoryInterface::class);
         $propertyMetadataFactoryProphecy->create(Dummy::class, 'name', [])->willReturn((new ApiProperty())->withBuiltinTypes([new Type(Type::BUILTIN_TYPE_STRING)])->withDescription('')->withReadable(false)->withWritable(true));
         $propertyMetadataFactoryProphecy->create(Dummy::class, 'relatedDummy', [])->willReturn((new ApiProperty())->withBuiltinTypes([$relatedDummyType])->withReadable(false)->withWritable(true)->withReadableLink(false)->withWritableLink(true));
         $propertyMetadataFactoryProphecy->create(Dummy::class, 'relatedDummies', [])->willReturn((new ApiProperty())->withBuiltinTypes([$relatedDummiesType])->withReadable(false)->withWritable(true)->withReadableLink(false)->withWritableLink(true));
+        $propertyMetadataFactoryProphecy->create(Dummy::class, 'relatedDummiesWithUnionTypes', [])->willReturn((new ApiProperty())->withBuiltinTypes([$relatedDummiesWithUnionTypesIntType, $relatedDummiesWithUnionTypesFloatType])->withReadable(false)->withWritable(true)->withReadableLink(false)->withWritableLink(true));
 
         $iriConverterProphecy = $this->prophesize(IriConverterInterface::class);
 
@@ -908,6 +914,8 @@ public function testDenormalizeWritableLinks(): void
         $serializerProphecy->willImplement(DenormalizerInterface::class);
         $serializerProphecy->denormalize(['foo' => 'bar'], RelatedDummy::class, null, Argument::type('array'))->willReturn($relatedDummy1);
         $serializerProphecy->denormalize(['bar' => 'baz'], RelatedDummy::class, null, Argument::type('array'))->willReturn($relatedDummy2);
+        $serializerProphecy->denormalize(['bar' => 'qux'], RelatedDummy::class, null, Argument::type('array'))->willReturn($relatedDummy3);
+        $serializerProphecy->denormalize(['bar' => 'quux'], RelatedDummy::class, null, Argument::type('array'))->willReturn($relatedDummy4);
 
         $normalizer = $this->getMockForAbstractClass(AbstractItemNormalizer::class, [
             $propertyNameCollectionFactoryProphecy->reveal(),
@@ -930,6 +938,7 @@ public function testDenormalizeWritableLinks(): void
         $propertyAccessorProphecy->setValue($actual, 'name', 'foo')->shouldHaveBeenCalled();
         $propertyAccessorProphecy->setValue($actual, 'relatedDummy', $relatedDummy1)->shouldHaveBeenCalled();
         $propertyAccessorProphecy->setValue($actual, 'relatedDummies', [$relatedDummy2])->shouldHaveBeenCalled();
+        $propertyAccessorProphecy->setValue($actual, 'relatedDummiesWithUnionTypes', [0 => $relatedDummy3, 1. => $relatedDummy4])->shouldHaveBeenCalled();
     }
 
     public function testBadRelationType(): void
@@ -1220,6 +1229,11 @@ public function testDenormalizeBadKeyType(): void
                     'bar' => 'baz',
                 ],
             ],
+            'relatedDummiesWithUnionTypes' => [
+                'a' => [
+                    'bar' => 'baz',
+                ],
+            ],
         ];
 
         $propertyNameCollectionFactoryProphecy = $this->prophesize(PropertyNameCollectionFactoryInterface::class);
diff --git a/src/Serializer/Tests/Fixtures/ApiResource/Dummy.php b/src/Serializer/Tests/Fixtures/ApiResource/Dummy.php
index ea62a525ba..a85f785d10 100644
--- a/src/Serializer/Tests/Fixtures/ApiResource/Dummy.php
+++ b/src/Serializer/Tests/Fixtures/ApiResource/Dummy.php
@@ -85,6 +85,13 @@ class Dummy
 
     public Collection|iterable $relatedDummies;
 
+    /**
+     * @phpstan-ignore-next-line
+     *
+     * @var Collection<int|float, RelatedDummy>
+     */
+    public Collection $relatedDummiesWithUnionTypes;
+
     /**
      * @var array|null serialize data
      */
@@ -107,6 +114,7 @@ public static function staticMethod(): void
     public function __construct()
     {
         $this->relatedDummies = new ArrayCollection();
+        $this->relatedDummiesWithUnionTypes = new ArrayCollection();
     }
 
     public function getId()