opening task in web - error ?

Server Traceback (most recent call last):
  File "/srv/openerp/instances/trunk/openerp-web/addons/web/common/openerplib/main.py", line 257, in send
    result = openerp.netsvc.dispatch_rpc(service_name, method, args)
  File "/home/srv/openerp/instances/trunk/openobject-server/openerp/netsvc.py", line 325, in dispatch_rpc
    result = ExportService.getService(service_name).dispatch(method, params)
  File "/home/srv/openerp/instances/trunk/openobject-server/openerp/service/web_services.py", line 580, in dispatch
    res = fn(db, uid, *params)
  File "/home/srv/openerp/instances/trunk/openobject-server/openerp/osv/osv.py", line 120, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/home/srv/openerp/instances/trunk/openobject-server/openerp/osv/osv.py", line 172, in execute
    res = self.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/srv/openerp/instances/trunk/openobject-addons/audittrail/audittrail.py", line 495, in execute_cr
    return fct_src(cr, uid, model, method, *args)
  File "/home/srv/openerp/instances/trunk/openobject-server/openerp/osv/osv.py", line 163, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/srv/openerp/instances/trunk/openobject-addons/base_calendar/base_calendar.py", line 1475, in read_group
    re.get('__context').update({'virtual_id' : virtual_id})
AttributeError: 'NoneType' object has no attribute 'update'