diff --git a/Tests/Fixtures/AbstractNormalizerDummy.php b/Tests/Fixtures/AbstractNormalizerDummy.php
index ae3b411b..a3d28c6c 100644
--- a/Tests/Fixtures/AbstractNormalizerDummy.php
+++ b/Tests/Fixtures/AbstractNormalizerDummy.php
@@ -31,14 +31,14 @@ public function getAllowedAttributes($classOrObject, array $context, bool $attri
     /**
      * {@inheritdoc}
      */
-    public function normalize($object, string $format = null, array $context = [])
+    public function normalize($object, ?string $format = null, array $context = [])
     {
     }
 
     /**
      * {@inheritdoc}
      */
-    public function supportsNormalization($data, string $format = null): bool
+    public function supportsNormalization($data, ?string $format = null): bool
     {
         return true;
     }
@@ -46,14 +46,14 @@ public function supportsNormalization($data, string $format = null): bool
     /**
      * {@inheritdoc}
      */
-    public function denormalize($data, string $type, string $format = null, array $context = [])
+    public function denormalize($data, string $type, ?string $format = null, array $context = [])
     {
     }
 
     /**
      * {@inheritdoc}
      */
-    public function supportsDenormalization($data, string $type, string $format = null): bool
+    public function supportsDenormalization($data, string $type, ?string $format = null): bool
     {
         return true;
     }
diff --git a/Tests/Fixtures/DenormalizableDummy.php b/Tests/Fixtures/DenormalizableDummy.php
index e7c03e3d..3170e361 100644
--- a/Tests/Fixtures/DenormalizableDummy.php
+++ b/Tests/Fixtures/DenormalizableDummy.php
@@ -16,7 +16,7 @@
 
 class DenormalizableDummy implements DenormalizableInterface
 {
-    public function denormalize(DenormalizerInterface $denormalizer, $data, string $format = null, array $context = [])
+    public function denormalize(DenormalizerInterface $denormalizer, $data, ?string $format = null, array $context = [])
     {
     }
 }
diff --git a/Tests/Fixtures/Dummy.php b/Tests/Fixtures/Dummy.php
index da040059..db09a40b 100644
--- a/Tests/Fixtures/Dummy.php
+++ b/Tests/Fixtures/Dummy.php
@@ -23,7 +23,7 @@ class Dummy implements NormalizableInterface, DenormalizableInterface
     public $baz;
     public $qux;
 
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, ?string $format = null, array $context = [])
     {
         return [
             'foo' => $this->foo,
@@ -33,7 +33,7 @@ public function normalize(NormalizerInterface $normalizer, string $format = null
         ];
     }
 
-    public function denormalize(DenormalizerInterface $denormalizer, $data, string $format = null, array $context = [])
+    public function denormalize(DenormalizerInterface $denormalizer, $data, ?string $format = null, array $context = [])
     {
         $this->foo = $data['foo'];
         $this->bar = $data['bar'];
diff --git a/Tests/Fixtures/DummyString.php b/Tests/Fixtures/DummyString.php
index 056de300..4a4bef8a 100644
--- a/Tests/Fixtures/DummyString.php
+++ b/Tests/Fixtures/DummyString.php
@@ -22,7 +22,7 @@ class DummyString implements DenormalizableInterface
     /** @var string $value */
     public $value;
 
-    public function denormalize(DenormalizerInterface $denormalizer, $data, string $format = null, array $context = [])
+    public function denormalize(DenormalizerInterface $denormalizer, $data, ?string $format = null, array $context = [])
     {
         $this->value = $data;
     }
diff --git a/Tests/Fixtures/EnvelopeNormalizer.php b/Tests/Fixtures/EnvelopeNormalizer.php
index 1492d5d0..f321d55a 100644
--- a/Tests/Fixtures/EnvelopeNormalizer.php
+++ b/Tests/Fixtures/EnvelopeNormalizer.php
@@ -20,7 +20,7 @@ class EnvelopeNormalizer implements NormalizerInterface
 {
     private $serializer;
 
-    public function normalize($envelope, string $format = null, array $context = []): array
+    public function normalize($envelope, ?string $format = null, array $context = []): array
     {
         $xmlContent = $this->serializer->serialize($envelope->message, 'xml');
 
@@ -31,7 +31,7 @@ public function normalize($envelope, string $format = null, array $context = [])
         ];
     }
 
-    public function supportsNormalization($data, string $format = null, array $context = []): bool
+    public function supportsNormalization($data, ?string $format = null, array $context = []): bool
     {
         return $data instanceof EnvelopeObject;
     }
diff --git a/Tests/Fixtures/EnvelopedMessageNormalizer.php b/Tests/Fixtures/EnvelopedMessageNormalizer.php
index dfdec91b..68a5b947 100644
--- a/Tests/Fixtures/EnvelopedMessageNormalizer.php
+++ b/Tests/Fixtures/EnvelopedMessageNormalizer.php
@@ -18,14 +18,14 @@
  */
 class EnvelopedMessageNormalizer implements NormalizerInterface
 {
-    public function normalize($message, string $format = null, array $context = []): array
+    public function normalize($message, ?string $format = null, array $context = []): array
     {
         return [
             'text' => $message->text,
         ];
     }
 
-    public function supportsNormalization($data, string $format = null, array $context = []): bool
+    public function supportsNormalization($data, ?string $format = null, array $context = []): bool
     {
         return $data instanceof EnvelopedMessage;
     }
diff --git a/Tests/Fixtures/NormalizableTraversableDummy.php b/Tests/Fixtures/NormalizableTraversableDummy.php
index 55b4402b..26bbae37 100644
--- a/Tests/Fixtures/NormalizableTraversableDummy.php
+++ b/Tests/Fixtures/NormalizableTraversableDummy.php
@@ -18,7 +18,7 @@
 
 class NormalizableTraversableDummy extends TraversableDummy implements NormalizableInterface, DenormalizableInterface
 {
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, ?string $format = null, array $context = [])
     {
         return [
             'foo' => 'normalizedFoo',
@@ -26,7 +26,7 @@ public function normalize(NormalizerInterface $normalizer, string $format = null
         ];
     }
 
-    public function denormalize(DenormalizerInterface $denormalizer, $data, string $format = null, array $context = [])
+    public function denormalize(DenormalizerInterface $denormalizer, $data, ?string $format = null, array $context = [])
     {
         return [
             'foo' => 'denormalizedFoo',
diff --git a/Tests/Fixtures/NotNormalizableDummy.php b/Tests/Fixtures/NotNormalizableDummy.php
index 8bb655db..86ee6dbe 100644
--- a/Tests/Fixtures/NotNormalizableDummy.php
+++ b/Tests/Fixtures/NotNormalizableDummy.php
@@ -24,7 +24,7 @@ public function __construct()
     {
     }
 
-    public function denormalize(DenormalizerInterface $denormalizer, $data, string $format = null, array $context = [])
+    public function denormalize(DenormalizerInterface $denormalizer, $data, ?string $format = null, array $context = [])
     {
         throw new NotNormalizableValueException();
     }
diff --git a/Tests/Fixtures/ScalarDummy.php b/Tests/Fixtures/ScalarDummy.php
index ffe4ee65..0704db31 100644
--- a/Tests/Fixtures/ScalarDummy.php
+++ b/Tests/Fixtures/ScalarDummy.php
@@ -21,12 +21,12 @@ class ScalarDummy implements NormalizableInterface, DenormalizableInterface
     public $foo;
     public $xmlFoo;
 
-    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = [])
+    public function normalize(NormalizerInterface $normalizer, ?string $format = null, array $context = [])
     {
         return 'xml' === $format ? $this->xmlFoo : $this->foo;
     }
 
-    public function denormalize(DenormalizerInterface $denormalizer, $data, string $format = null, array $context = [])
+    public function denormalize(DenormalizerInterface $denormalizer, $data, ?string $format = null, array $context = [])
     {
         if ('xml' === $format) {
             $this->xmlFoo = $data;
diff --git a/Tests/Normalizer/AbstractObjectNormalizerTest.php b/Tests/Normalizer/AbstractObjectNormalizerTest.php
index 5adc3d4e..5243eb2f 100644
--- a/Tests/Normalizer/AbstractObjectNormalizerTest.php
+++ b/Tests/Normalizer/AbstractObjectNormalizerTest.php
@@ -559,12 +559,12 @@ public function testProvidingContextCacheKeyGeneratesSameChildContextCacheKey()
         $normalizer = new class() extends AbstractObjectNormalizerDummy {
             public $childContextCacheKey;
 
-            protected function extractAttributes(object $object, string $format = null, array $context = []): array
+            protected function extractAttributes(object $object, ?string $format = null, array $context = []): array
             {
                 return array_keys((array) $object);
             }
 
-            protected function getAttributeValue(object $object, string $attribute, string $format = null, array $context = [])
+            protected function getAttributeValue(object $object, string $attribute, ?string $format = null, array $context = [])
             {
                 return $object->{$attribute};
             }
@@ -599,12 +599,12 @@ public function testChildContextKeepsOriginalContextCacheKey()
         $normalizer = new class() extends AbstractObjectNormalizerDummy {
             public $childContextCacheKey;
 
-            protected function extractAttributes(object $object, string $format = null, array $context = []): array
+            protected function extractAttributes(object $object, ?string $format = null, array $context = []): array
             {
                 return array_keys((array) $object);
             }
 
-            protected function getAttributeValue(object $object, string $attribute, string $format = null, array $context = [])
+            protected function getAttributeValue(object $object, string $attribute, ?string $format = null, array $context = [])
             {
                 return $object->{$attribute};
             }
@@ -634,12 +634,12 @@ public function testChildContextCacheKeyStaysFalseWhenOriginalCacheKeyIsFalse()
         $normalizer = new class() extends AbstractObjectNormalizerDummy {
             public $childContextCacheKey;
 
-            protected function extractAttributes(object $object, string $format = null, array $context = []): array
+            protected function extractAttributes(object $object, ?string $format = null, array $context = []): array
             {
                 return array_keys((array) $object);
             }
 
-            protected function getAttributeValue(object $object, string $attribute, string $format = null, array $context = [])
+            protected function getAttributeValue(object $object, string $attribute, ?string $format = null, array $context = [])
             {
                 return $object->{$attribute};
             }