diff --git a/complaint.py b/complaint.py
index 7e5d4c7382..343b882c19 100644
--- a/complaint.py
+++ b/complaint.py
@@ -618,7 +618,7 @@ class Action(ModelSQL, ModelView):
 
 
 class _Action_Line:
-
+    __slots__ = ()
     _states = {
         'readonly': (
             (Eval('complaint_state') != 'draft')