diff --git a/invoice.py b/invoice.py
index 00bc2ad..f5a70cb 100644
--- a/invoice.py
+++ b/invoice.py
@@ -17,9 +17,7 @@ class InvoiceLine(metaclass=PoolMeta):
             cls.unit.states['required'],
             And(
                 Eval('type') == 'line',
-                Eval('quantity', None) != None,
-                ),
-            )
+                Bool(Eval('quantity', None))))
         cls.unit.depends.extend(['type', 'quantity'])
 
 # end Invoice