django - Django 1.6.5 と factory_boy の例外

Question

テスト目的で、単純な django アプリケーションで factory_boy を使用することにしました。しかし、単純な例に問題がありました。これが私の簡単なテストのコードです。

from django.utils import unittest
from ..models import Server, ServerAddress, L2TPServer, serialize_open_vpn_server_json

from factory import django as django_factory


class SshOpenVpnServerFactory(django_factory.DjangoModelFactory):
    class Meta:
        model = L2TPServer
        django_get_or_create = ('name', 'address')

    name = 'Hello'
    address = 'Nono'


class ServersTestCase(unittest.TestCase):

    def test_serialize_server_info(self):
        print Server.objects.all()
        SshOpenVpnServerFactory.build()

test-runner がこのテストを実行すると、次のエラーが発生します。

Traceback (most recent call last):
  File "/Users/green/Development/Wasel/experimental/wasel_services/packages/waselcore/backend/tests/test_models.py", line 20, in test_serialize_server_info
    SshOpenVpnServerFactory.build()
  File "/Users/green/Development/Wasel/experimental/env/wasel_sevices/lib/python2.7/site-packages/factory/base.py", line 504, in build
    attrs = cls.attributes(create=False, extra=kwargs)
  File "/Users/green/Development/Wasel/experimental/env/wasel_sevices/lib/python2.7/site-packages/factory/base.py", line 365, in attributes
    force_sequence=force_sequence,
  File "/Users/green/Development/Wasel/experimental/env/wasel_sevices/lib/python2.7/site-packages/factory/containers.py", line 265, in build
    sequence = self.factory._generate_next_sequence()
  File "/Users/green/Development/Wasel/experimental/env/wasel_sevices/lib/python2.7/site-packages/factory/base.py", line 338, in _generate_next_sequence
    cls._setup_counter()
  File "/Users/green/Development/Wasel/experimental/env/wasel_sevices/lib/python2.7/site-packages/factory/base.py", line 318, in _setup_counter
    first_seq = cls._setup_next_sequence()
  File "/Users/green/Development/Wasel/experimental/env/wasel_sevices/lib/python2.7/site-packages/factory/django.py", line 83, in _setup_next_sequence
    manager = cls._get_manager(model)
  File "/Users/green/Development/Wasel/experimental/env/wasel_sevices/lib/python2.7/site-packages/factory/django.py", line 76, in _get_manager
    return target_class.objects
AttributeError: 'NoneType' object has no attribute 'objects'

どこが間違っていますか？factory_boy は django 1.6.5 をサポートしていますか?