diff --git a/invoice.py b/invoice.py
index 16579d9..3fdfa64 100644
--- a/invoice.py
+++ b/invoice.py
@@ -145,7 +145,7 @@ class Invoice(metaclass=PoolMeta):
     @classmethod
     def __setup__(cls):
         super(Invoice, cls).__setup__()
-        cls._check_modify_exclude.append('invoice_facturae')
+        cls._check_modify_exclude.add('invoice_facturae')
         cls._buttons.update({
                 'generate_facturae_wizard': {
                     'invisible': ((Eval('type') != 'out')