diff --git a/src/Normalizer/MenuLinkNormalizer.php b/src/Normalizer/MenuLinkNormalizer.php
index 6eeea63e4d249163554aa976b8d58c79d2c86c99..8920bce0bdc5dcb647d11d7f1456fbc18a6342ed 100644
--- a/src/Normalizer/MenuLinkNormalizer.php
+++ b/src/Normalizer/MenuLinkNormalizer.php
@@ -20,7 +20,14 @@ class MenuLinkNormalizer extends NormalizerBase {
   /**
    * {@inheritdoc}
    */
-  public function normalize($object, $format = NULL, array $context = []) {
+  public function supportsNormalization($data, ?string $format = null, array $context = []): bool {
+    return $data instanceof MenuLinkInterface;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function normalize($object, ?string $format = NULL, array $context = []): array {
     /** @var \Drupal\Core\Menu\MenuLinkInterface $object */
     return [
       'id' => $object->getPluginId(),
diff --git a/src/Normalizer/MenuLinkTreeNormalizer.php b/src/Normalizer/MenuLinkTreeNormalizer.php
index 87350cfe411a4902532a2f5be1980d90a151de51..ccc280715bfbbd2dfeb21d9291544f86d00d3cf3 100644
--- a/src/Normalizer/MenuLinkTreeNormalizer.php
+++ b/src/Normalizer/MenuLinkTreeNormalizer.php
@@ -20,7 +20,14 @@ class MenuLinkTreeNormalizer extends NormalizerBase {
   /**
    * {@inheritdoc}
    */
-  public function normalize($object, $format = NULL, array $context = []) {
+  public function supportsNormalization($data, ?string $format = null, array $context = []): bool {
+    return $data instanceof MenuLinkTreeElement;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function normalize($object, ?string $format = NULL, array $context = []): array {
     /** @var \Drupal\Core\Menu\MenuLinkTreeElement $object */
     return [
       'link' => $this->serializer->normalize($object->link, $format, $context),