diff --git a/product/ERP5Type/Accessor/Base.py b/product/ERP5Type/Accessor/Base.py
index 14b0681b7d1e4a85bd2aecadad8e7eeb327fd9ea..03ca8d4ca50e6ac275afd41d31aeb26fa6abef0e 100644
--- a/product/ERP5Type/Accessor/Base.py
+++ b/product/ERP5Type/Accessor/Base.py
@@ -188,4 +188,4 @@ class Tester(Method):
       self._storage_id = storage_id
 
     def __call__(self, instance, *args, **kw):
-      return getattr(aq_base(instance), self._storage_id, None) is not None
+      return getattr(aq_base(instance), self._storage_id, None) not in self._null