diff --git a/addons/sale/models/sale.py b/addons/sale/models/sale.py
index c53a777a7c7..4858afdaa19 100644
--- a/addons/sale/models/sale.py
+++ b/addons/sale/models/sale.py
@@ -424,7 +424,7 @@ class SaleOrder(models.Model):
 
             if references.get(invoices.get(group_key)):
                 if order not in references[invoices[group_key]]:
-                    references[invoice] = references[invoice] | order
+                    references[invoices[group_key]] |= order
 
         if not invoices:
             raise UserError(_('There is no invoiceable line.'))