diff --git a/sale.py b/sale.py
index 68dac98783..311744cca1 100644
--- a/sale.py
+++ b/sale.py
@@ -738,7 +738,9 @@ class Sale(Workflow, ModelSQL, ModelView):
             return
 
         invoice = self._get_invoice_sale(invoice_type)
-        invoice.lines = list(chain.from_iterable(invoice_lines.itervalues()))
+        invoice.lines = ((list(invoice.lines)
+                if hasattr(invoice, 'lines') else [])
+            + list(chain.from_iterable(invoice_lines.itervalues())))
         invoice.save()
 
         with Transaction().set_user(0, set_context=True):