diff --git a/quality.py b/quality.py
index 9b5823d..05c38ca 100644
--- a/quality.py
+++ b/quality.py
@@ -62,6 +62,7 @@ class QualityTest(metaclass=PoolMeta):
 
 
 class TestLineMixin(Model):
+    __slots__ = ()
     product = fields.Function(fields.Many2One('product.product', 'Product', select=True),
         'get_product', searcher='search_product')
 
diff --git a/wine.py b/wine.py
index 39da3ad..2ad36ce 100644
--- a/wine.py
+++ b/wine.py
@@ -13,6 +13,7 @@ _WINE_DIGITS = 4
 
 
 class WineMixin(Model):
+    __slots__ = ()
     wine_quality_comment = fields.Function(fields.Text('Wine Quality Comments'),
         'get_wine_quality_comment')