diff --git a/src/HybridSession.php b/src/HybridSession.php
index 06bcef2..9d8e6eb 100644
--- a/src/HybridSession.php
+++ b/src/HybridSession.php
@@ -122,7 +122,7 @@ public function gc(int $maxlifetime): int|false
      *
      * @param string $key Desired session key
      */
-    public static function init(string $key = null)
+    public static function init(?string $key = null)
     {
         $instance = Injector::inst()->get(__CLASS__);