diff --git a/openerp/addons/base/res/res_user.py b/openerp/addons/base/res/res_user.py
index 8a76ba9a6f4..55451f1bb50 100644
--- a/openerp/addons/base/res/res_user.py
+++ b/openerp/addons/base/res/res_user.py
@@ -582,8 +582,7 @@ class groups2(osv.osv): ##FIXME: Is there a reason to inherit this object ?
         if group_users:
             user_names = [user.name for user in self.pool.get('res.users').browse(cr, uid, group_users, context=context)]
             if len(user_names) >= 5:
-                user_names = user_names[:5]
-                user_names += '...'
+                user_names = user_names[:5] + ['...']
             raise osv.except_osv(_('Warning !'),
                         _('Group(s) cannot be deleted, because some user(s) still belong to them: %s !') % \
                             ', '.join(user_names))