diff --git a/src/Symfony/Component/Serializer/Normalizer/GetSetMethodNormalizer.php b/src/Symfony/Component/Serializer/Normalizer/GetSetMethodNormalizer.php
index 348ab78bb267..b3111eaa1820 100644
--- a/src/Symfony/Component/Serializer/Normalizer/GetSetMethodNormalizer.php
+++ b/src/Symfony/Component/Serializer/Normalizer/GetSetMethodNormalizer.php
@@ -68,7 +68,7 @@ public function normalize($object, $format = null, array $context = array())
                 }
 
                 $attributeValue = $method->invoke($object);
-                if (array_key_exists($attributeName, $this->callbacks)) {
+                if (isset($this->callbacks[$attributeName])) {
                     $attributeValue = call_user_func($this->callbacks[$attributeName], $attributeValue);
                 }
                 if (null !== $attributeValue && !is_scalar($attributeValue)) {
diff --git a/src/Symfony/Component/Serializer/Normalizer/PropertyNormalizer.php b/src/Symfony/Component/Serializer/Normalizer/PropertyNormalizer.php
index b54187abb298..5acadde2e7e3 100644
--- a/src/Symfony/Component/Serializer/Normalizer/PropertyNormalizer.php
+++ b/src/Symfony/Component/Serializer/Normalizer/PropertyNormalizer.php
@@ -64,7 +64,7 @@ public function normalize($object, $format = null, array $context = array())
 
             $attributeValue = $property->getValue($object);
 
-            if (array_key_exists($property->name, $this->callbacks)) {
+            if (isset($this->callbacks[$property->name])) {
                 $attributeValue = call_user_func($this->callbacks[$property->name], $attributeValue);
             }
             if (null !== $attributeValue && !is_scalar($attributeValue)) {