diff --git a/trytond/trytond/model/modelstorage.py b/trytond/trytond/model/modelstorage.py
index f0e6279315..51cb859291 100644
--- a/trytond/trytond/model/modelstorage.py
+++ b/trytond/trytond/model/modelstorage.py
@@ -1113,7 +1113,8 @@ class ModelStorage(Model):
         to_create, to_create_translations = [], []
         to_write, to_write_translations = [], []
         languages = set()
-        while len(data):
+        data = data.copy()
+        while data:
             (row, _, translate) = \
                 process_lines(data, [], fields_def)
             if dispatch(to_create, to_write, row):