diff --git a/stock.py b/stock.py
index ff6bd785dc..0e60dca5dc 100644
--- a/stock.py
+++ b/stock.py
@@ -98,6 +98,7 @@ class Package(metaclass=PoolMeta):
 
 
 class ShippingUPSMixin:
+    __slots__ = ()
 
     def validate_packing_ups(self):
         warehouse = self.shipping_warehouse