diff --git a/addons/account/account_move_line.py b/addons/account/account_move_line.py
index 75f6198c7d2..95419811206 100644
--- a/addons/account/account_move_line.py
+++ b/addons/account/account_move_line.py
@@ -782,7 +782,7 @@ class account_move_line(osv.osv):
             ret_line = {
                 'id': line.id,
                 'name': line.name != '/' and line.move_id.name + ': ' + line.name or line.move_id.name,
-                'ref': line.move_id.ref,
+                'ref': line.move_id.ref or '',
                 'account_code': line.account_id.code,
                 'account_name': line.account_id.name,
                 'account_type': line.account_id.type,