diff --git a/invoice.py b/invoice.py
index 447bcd0cde..b936a9a404 100644
--- a/invoice.py
+++ b/invoice.py
@@ -9,6 +9,13 @@ class Invoice(ModelSQL, ModelView):
     sales = fields.Many2Many('sale.sale-account.invoice',
             'invoice', 'sale', 'Sales', readonly=True)
 
+    def copy(self, ids, default=None):
+        if default is None:
+            default = {}
+        default = default.copy()
+        default.setdefault('sales', False)
+        return super(Invoice, self).copy(ids, default=default)
+
 Invoice()
 
 
@@ -18,4 +25,11 @@ class InvoiceLine(ModelSQL, ModelView):
     sale_lines = fields.Many2Many('sale.line-account.invoice.line',
             'invoice_line', 'sale_line', 'Sale Lines', readonly=True)
 
+    def copy(self, ids, default=None):
+        if default is None:
+            default = {}
+        default = default.copy()
+        default.setdefault('sale_lines', False)
+        return super(InvoiceLine, self).copy(ids, default=default)
+
 InvoiceLine()