diff --git a/src/HttpRequest.php b/src/HttpRequest.php
index c8c0f7c..bc34252 100644
--- a/src/HttpRequest.php
+++ b/src/HttpRequest.php
@@ -55,7 +55,7 @@ class HttpRequest extends Collection
         return parent::get('data')->get($key, $default);
     }
 
-    function session(Session $value = null)
+    function session(?Session $value = null)
     {
         if ($value) {
             $this->_session = $value;