diff --git a/lbplanner/services/user/get_all_users.php b/lbplanner/services/user/get_all_users.php
index 5df4bc2..c5de61b 100644
--- a/lbplanner/services/user/get_all_users.php
+++ b/lbplanner/services/user/get_all_users.php
@@ -48,7 +48,7 @@ public static function get_all_users_parameters(): external_function_parameters
 
     /**
      * Gives back all users registered by the lbplanner app.
-     * @param string $vintage (optional) gives back all users with the given vintage
+     * @param ?string $vintage (optional) gives back all users with the given vintage
      * @throws moodle_exception
      * @throws dml_exception
      * @throws invalid_parameter_exception