diff --git a/trytond/model/modelstorage.py b/trytond/model/modelstorage.py
index 4ddb2a5f34..0c84aea3ee 100644
--- a/trytond/model/modelstorage.py
+++ b/trytond/model/modelstorage.py
@@ -758,6 +758,7 @@ class ModelStorage(Model):
 
         while len(datas):
             res = {}
+            warning = ''
             try:
                 (res, other, warning, translate) = \
                         process_liness(self, datas, [], fields_def)