diff --git a/commission.py b/commission.py
index 102fda76b9..552a2267b3 100644
--- a/commission.py
+++ b/commission.py
@@ -313,6 +313,8 @@ class Commission(ModelSQL, ModelView):
     def copy(cls, commissions, default=None):
         if default is None:
             default = {}
+        else:
+            default = default.copy()
         default.setdefault('invoice_line', None)
         return super(Commission, cls).copy(commissions, default=default)
 
diff --git a/invoice.py b/invoice.py
index bb41d97dba..93d37be1b8 100644
--- a/invoice.py
+++ b/invoice.py
@@ -196,6 +196,8 @@ class InvoiceLine(metaclass=PoolMeta):
     def copy(cls, lines, default=None):
         if default is None:
             default = {}
+        else:
+            default = default.copy()
         default.setdefault('commissions', None)
         default.setdefault('from_commissions', None)
         return super(InvoiceLine, cls).copy(lines, default=default)