diff --git a/account.py b/account.py
index 1dd7cdc694..a4a6c36920 100644
--- a/account.py
+++ b/account.py
@@ -37,5 +37,5 @@ class MoveLine(metaclass=PoolMeta):
             default = {}
         else:
             default = default.copy()
-        default.setdefault('dunnings')
+        default.setdefault('dunnings', None)
         return super(MoveLine, cls).copy(lines, default=default)