diff --git a/core/modules/jsonapi/src/Normalizer/EntityDenormalizerBase.php b/core/modules/jsonapi/src/Normalizer/EntityDenormalizerBase.php
index a905c609a92c20532583bdb9a84f03d87249c6bb..efdae5241dc26e8351e2088cae68679ed7036259 100644
--- a/core/modules/jsonapi/src/Normalizer/EntityDenormalizerBase.php
+++ b/core/modules/jsonapi/src/Normalizer/EntityDenormalizerBase.php
@@ -67,7 +67,7 @@ public function __construct(EntityTypeManagerInterface $entity_type_manager, Ent
   /**
    * {@inheritdoc}
    */
-  public function supportsNormalization($data, $format = NULL) {
+  public function supportsNormalization($data, $format = NULL): bool {
     return FALSE;
   }
 
diff --git a/core/modules/jsonapi/src/Serializer/Serializer.php b/core/modules/jsonapi/src/Serializer/Serializer.php
index 709cf770949c3f90aab4dfc799f4528ab335355b..cb45e8984cc3b43959eac50b414c781689a29763 100644
--- a/core/modules/jsonapi/src/Serializer/Serializer.php
+++ b/core/modules/jsonapi/src/Serializer/Serializer.php
@@ -80,7 +80,7 @@ public function denormalize($data, $type, $format = NULL, array $context = []) {
   /**
    * {@inheritdoc}
    */
-  public function supportsNormalization($data, $format = NULL, array $context = []) {
+  public function supportsNormalization($data, $format = NULL, array $context = []): bool {
     return $this->selfSupportsNormalization($data, $format, $context) || $this->fallbackNormalizer->supportsNormalization($data, $format, $context);
   }
 
diff --git a/core/modules/serialization/src/Normalizer/NormalizerBase.php b/core/modules/serialization/src/Normalizer/NormalizerBase.php
index f1539b62a631f54436a5aa589971d6fd0e353a55..f98e94e26bfae042743fce82ea0b6aed446be912 100644
--- a/core/modules/serialization/src/Normalizer/NormalizerBase.php
+++ b/core/modules/serialization/src/Normalizer/NormalizerBase.php
@@ -30,7 +30,7 @@ abstract class NormalizerBase implements SerializerAwareInterface, CacheableNorm
   /**
    * {@inheritdoc}
    */
-  public function supportsNormalization($data, $format = NULL) {
+  public function supportsNormalization($data, $format = NULL): bool {
     // If we aren't dealing with an object or the format is not supported return
     // now.
     if (!is_object($data) || !$this->checkFormat($format)) {
diff --git a/core/modules/serialization/tests/serialization_test/src/SerializationTestNormalizer.php b/core/modules/serialization/tests/serialization_test/src/SerializationTestNormalizer.php
index 35300d8a8cdcd3784ee55bdb4b54e11034a5ba3d..cc205e8cd7a9fa8f85a9017266ebc4957f2c8f44 100644
--- a/core/modules/serialization/tests/serialization_test/src/SerializationTestNormalizer.php
+++ b/core/modules/serialization/tests/serialization_test/src/SerializationTestNormalizer.php
@@ -47,7 +47,7 @@ public function normalize($object, $format = NULL, array $context = []) {
    * @return bool
    *   Returns TRUE if the normalizer can handle the request.
    */
-  public function supportsNormalization($data, $format = NULL) {
+  public function supportsNormalization($data, $format = NULL): bool {
     return static::$format === $format;
   }