module to add a field to partner database gives error KeyError: 'lang'

I tried to develop a small module in openerp v 7, but after the installation openerp gives me this error:
File "/usr/local/data/openerp/7.0/openerp/addons/web/http.py", line 204, in dispatch response["result"] = method(self, **self.params)
File "/usr/local/data/openerp/7.0/openerp/addons/web/controllers/main.py", line 867, in authenticate req.session.authenticate(db, login, password, env)
File "/usr/local/data/openerp/7.0/openerp/addons/web/session.py", line 118, in authenticate if uid: self.get_context()
File "/usr/local/data/openerp/7.0/openerp/addons/web/session.py", line 172, in get_context self._fix_lang(self.context)
File "/usr/local/data/openerp/7.0/openerp/addons/web/session.py", line 183, in _fix_lang lang = context['lang']
KeyError: 'lang'
Here are the source codes: **FILE __openerp__.py
** {
   'name': 'Psl1',
   'version': '0.8',
   'category': 'Tools',
   'description': "prove psl aggiunta campo",
   'author': 'RT',
   'website': 'progettols',
   depends': ['base'],
   'data': [],
   'demo': [],
   'test':[],
   'installable': True,
   'images': [],
}
**FILE __init__.py
** import psl1
**FILE psl1.py
** from openerp.osv import osv
from openerp.osv import fields
class res_partner(osv.osv):
   _name = 'res.partner'
   __inherit = 'res.partner'
   _columns = {
   'specialcode':fields.char('Internal specialcode',size=64),
   }
could someone help me to understand the error ?