diff --git a/src/AbstractSerializer.php b/src/AbstractSerializer.php
index 6e343cc..6f5825f 100644
--- a/src/AbstractSerializer.php
+++ b/src/AbstractSerializer.php
@@ -41,7 +41,7 @@ public function getId($model)
     /**
      * {@inheritdoc}
      */
-    public function getAttributes($model, array $fields = null)
+    public function getAttributes($model, ?array $fields = null)
     {
         return [];
     }
diff --git a/src/SerializerInterface.php b/src/SerializerInterface.php
index 96730fa..f6fd085 100644
--- a/src/SerializerInterface.php
+++ b/src/SerializerInterface.php
@@ -39,7 +39,7 @@ public function getId($model);
      *
      * @return array
      */
-    public function getAttributes($model, array $fields = null);
+    public function getAttributes($model, ?array $fields = null);
 
     /**
      * Get the links array.