diff --git a/sale.py b/sale.py
index bcf7d3201a..40d8ac3edd 100644
--- a/sale.py
+++ b/sale.py
@@ -1675,8 +1675,9 @@ class ReturnSale(Wizard):
         return_sales = Sale.copy(sales)
         for sale in return_sales:
             for line in sale.lines:
-                line.quantity *= -1
-                line.save()
+                if line.type == 'line':
+                    line.quantity *= -1
+                    line.save()
         data = {'res_id': [s.id for s in return_sales]}
         if len(return_sales) == 1:
             action['views'].reverse()