diff --git a/lbplanner/services/user/get_all_users.php b/lbplanner/services/user/get_all_users.php
index 442da81f..c5de61b4 100644
--- a/lbplanner/services/user/get_all_users.php
+++ b/lbplanner/services/user/get_all_users.php
@@ -42,18 +42,18 @@ class user_get_all_users extends external_api {
      */
     public static function get_all_users_parameters(): external_function_parameters {
         return new external_function_parameters([
-            'vintage' => new external_value(PARAM_TEXT, 'The vintage to filter the users by', VALUE_DEFAULT, null),
+            'vintage' => new external_value(PARAM_TEXT, 'The vintage to filter the users by', VALUE_DEFAULT, null, NULL_ALLOWED),
         ]);
     }
 
     /**
      * 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
      */
-    public static function get_all_users(string $vintage): array {
+    public static function get_all_users(?string $vintage): array {
         global $DB, $USER;
 
         self::validate_parameters(self::get_all_users_parameters(), ['vintage' => $vintage]);