using it with forms

Hello I try ti ose it with modelforms and get error
```
python2.7/site-packages/django/forms/models.py", line 277, in __new__
    raise FieldError(message)
django.core.exceptions.FieldError: Unknown field(s) (surname,

```
this is models.py 
`   surname = xml_models.CharField(max_length=32, verbose_name=u'Фамилия', db_index=True, xpath='surname')`

how to fix this?