diff --git a/ParameterBag.php b/ParameterBag.php
index e1f89d69e..b542292bc 100644
--- a/ParameterBag.php
+++ b/ParameterBag.php
@@ -39,7 +39,7 @@ public function __construct(array $parameters = [])
      *
      * @return array
      */
-    public function all(/* string $key = null */)
+    public function all(/* ?string $key = null */)
     {
         $key = \func_num_args() > 0 ? func_get_arg(0) : null;