Hi i want to export the sales, but i have this bug

Client Traceback (most recent call last):
  File "/opt/openerp/server/openerp/addons/web/http.py", line 204, in dispatch
    response["result"] = method(self, **self.params)
  File "/opt/openerp/server/openerp/addons/web/controllers/main.py", line 1495, in get_fields
    name = parent_name + (parent_name and '/' or '') + field['string']
KeyError: 'string'

an idea ?