diff --git a/libraries/classes/Server/Privileges.php b/libraries/classes/Server/Privileges.php
index ae19cd5838..19d2a49372 100644
--- a/libraries/classes/Server/Privileges.php
+++ b/libraries/classes/Server/Privileges.php
@@ -590,6 +590,7 @@ class Privileges
      */
     public function setUserGroup($username, $userGroup)
     {
+        $userGroup = is_null($userGroup) ? '' : $userGroup;
         $relation = new Relation();
         $cfgRelation = $relation->getRelationsParam();
         if (empty($cfgRelation['db']) || empty($cfgRelation['users']) || empty($cfgRelation['usergroups'])) {