diff -r 656db55c90c0 trytond/trytond/modules/sale/product.py
--- a/trytond/trytond/modules/sale/product.py  Sat Jan 26 01:12:51 2019 +0100
+++ b/trytond/trytond/modules/sale/product.py  Wed Feb 06 16:31:25 2019 +0100
@@ -1,7 +1,7 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 import datetime
-
+from decimal import Decimal
 from trytond.model import fields
 from trytond.pyson import Eval
 from trytond.pool import Pool, PoolMeta
--- a/trytond/trytond/modules/sale/product.py	Tue Oct 01 22:23:55 2019 +0200
+++ b/trytond/trytond/modules/sale/product.py	Tue Oct 22 14:52:38 2019 +0200
@@ -119,7 +119,7 @@
         user = User(Transaction().user)

         for product in products:
-            prices[product.id] = product.list_price_used
+            prices[product.id] = product.list_price_used or Decimal(0)
             if uom and product.default_uom.category == uom.category:
                 prices[product.id] = Uom.compute_price(
                     product.default_uom, prices[product.id], uom)