Traceback (most recent call last):
  File "/home/gnuthink/openerp/empresa/server/bin/netsvc.py", line 489, in dispatch
    result = ExportService.getService(service_name).dispatch(method, auth, params)
  File "/home/gnuthink/openerp/empresa/server/bin/service/web_services.py", line 599, in dispatch
    res = fn(db, uid, *params)
  File "/home/gnuthink/openerp/empresa/server/bin/osv/osv.py", line 122, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/home/gnuthink/openerp/empresa/server/bin/osv/osv.py", line 176, in execute
    res = self.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/home/gnuthink/openerp/empresa/server/bin/osv/osv.py", line 167, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/home/gnuthink/openerp/empresa/addons/account_voucher/account_voucher.py", line 541, in onchange_date
    if context.get('invoice_id', False):
AttributeError: 'NoneType' object has no attribute 'get'