diff --git a/tryton/common/domain_inversion.py b/tryton/common/domain_inversion.py
index 7f2ed03e1f..620ed8d35e 100644
--- a/tryton/common/domain_inversion.py
+++ b/tryton/common/domain_inversion.py
@@ -370,9 +370,8 @@ class Or(And):
     def inverse(self, symbol, context):
         result = []
         known_variables = set(context.keys())
-        if (symbol not in self.variables
-                and not known_variables >= self.variables):
-            # In this case we don't know anything about this OR part, we
+        if not known_variables >= (self.variables - {symbol}):
+            # In this case we don't know enough about this OR part, we
             # consider it to be True (because people will have the constraint
             # on this part later).
             return True