diff --git a/components/serializer.rst b/components/serializer.rst
index 5f150a212b3..e7ed6f12550 100644
--- a/components/serializer.rst
+++ b/components/serializer.rst
@@ -625,7 +625,7 @@ defines a ``Person`` entity with a ``firstName`` property:
 This custom mapping is used to convert property names when serializing and
 deserializing objects::
 
-    $serialized = $serializer->serialize(new Person("Kévin"));
+    $serialized = $serializer->serialize(new Person("Kévin"), 'json');
     // {"customer_name": "Kévin"}
 
 Serializing Boolean Attributes
@@ -1450,7 +1450,7 @@ and ``BitBucketCodeRepository`` classes:
 
 Once configured, the serializer uses the mapping to pick the correct class::
 
-    $serialized = $serializer->serialize(new GitHubCodeRepository());
+    $serialized = $serializer->serialize(new GitHubCodeRepository(), 'json');
     // {"type": "github"}
 
     $repository = $serializer->deserialize($serialized, CodeRepository::class, 'json');