diff -r 56fa7c97d5b7 move.py
--- a/trytond/trytond/modules/stock/move.py	Thu Jun 06 10:24:15 2019 +0200
+++ b/trytond/trytond/modules/stock/move.py	Thu Jun 06 10:25:02 2019 +0200
@@ -1346,7 +1346,7 @@
                 location = line[0]
             key = tuple(line[1:-1])
             quantity = line[-1]
-            quantities[(location,) + key] += quantity
+            quantities[(location,) + key] += quantity or 0
             ids.add(id_getter(line))
             keys.add(key)