diff --git a/src/Rollout.php b/src/Rollout.php
index c5d8db5..8efee3e 100644
--- a/src/Rollout.php
+++ b/src/Rollout.php
@@ -29,7 +29,7 @@ public function __construct(StorageInterface $storage)
     {
         $this->storage = $storage;
         $this->groups = array(
-            'all' => function(RolloutUserInterface $user) { return $user !== null; }
+            'all' => function(?RolloutUserInterface $user) { return $user !== null; }
         );
     }