diff --git a/sale.py b/sale.py
index 0e5f6b0b5a..7ba50c3276 100644
--- a/sale.py
+++ b/sale.py
@@ -73,7 +73,7 @@ class Sale(metaclass=PoolMeta):
             default = {}
         else:
             default = default.copy()
-        default.setdefault('payments')
+        default.setdefault('payments', None)
         return super(Sale, cls).copy(sales, default=default)
 
     @classmethod