I have a django site which uses Haystack with the Xapian backend for search indexing. I've added a new field to one of the models being indexed, then added that field to the SearchIndex for that model. I've run:

`python manage.py update_index`

To update the index, but I'm getting the following error:

```
Traceback (most recent call last):
  File "manage.py", line 11, in <module>
    execute_manager(settings)
  File "/usr/local/lib/python2.6/dist-packages/django/core/management/__init__.py", line 438, in execute_manager
    utility.execute()
  File "/usr/local/lib/python2.6/dist-packages/django/core/management/__init__.py", line 379, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python2.6/dist-packages/django/core/management/base.py", line 191, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/usr/local/lib/python2.6/dist-packages/django/core/management/base.py", line 220, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python2.6/dist-packages/django_haystack-1.0.1_final-py2.6.egg/haystack/management/commands/update_index.py", line 51, in handle
    self.handle_app(None, **options)
  File "/usr/local/lib/python2.6/dist-packages/django_haystack-1.0.1_final-py2.6.egg/haystack/management/commands/update_index.py", line 107, in handle_app
    index.backend.update(index, small_cache_qs[start:end])
  File "/usr/local/lib/python2.6/dist-packages/xapian_haystack-1.1.3beta-py2.6.egg/xapian_backend.py", line 204, in update
    data = index.prepare(obj)
  File "/usr/local/lib/python2.6/dist-packages/django_haystack-1.0.1_final-py2.6.egg/haystack/indexes.py", line 102, in prepare
    self.prepared_data[field_name] = field.prepare(obj)
  File "/usr/local/lib/python2.6/dist-packages/django_haystack-1.0.1_final-py2.6.egg/haystack/fields.py", line 119, in prepare
    return self.convert(super(CharField, self).prepare(obj))
  File "/usr/local/lib/python2.6/dist-packages/django_haystack-1.0.1_final-py2.6.egg/haystack/fields.py", line 75, in prepare
    raise SearchFieldError("The model '%s' has an empty model_attr '%s' and doesn't allow a default or null value." % (repr(current_object), attr))
haystack.exceptions.SearchFieldError: The model 'None' has an empty model_attr 'address_county' and doesn't allow a default or null value.

```

The versions I'm using are django 1.2 and django-haystack 1.0.1. Upgrading these to the newest version isn't an option for me at the moment.