diff --git a/account.py b/account.py
index b4d6536bb8..aa7ff5b49e 100644
--- a/account.py
+++ b/account.py
@@ -9,6 +9,7 @@ from trytond.pyson import If, Eval, Bool
 def AccountTypeMixin(template=False):
 
     class Mixin:
+        __slots__ = ()
         unearned_revenue = fields.Boolean(
             "Unearned Revenue",
             domain=[