使用Django / South将ManyToManyField添加到现有模型中给我一个错误

这是我的生产模型（在selfcare.apps.support.models中）：

class Document(models.Model):
    CATEGORY_CHOICES = (
        ('HSI', _('Internet')),
        ('ITV', _('Television')),
        ('VOIP', _('Phone')),
        ('MAIL', _('Email')),
    )
    title = models.CharField(max_length=100, db_index=True)
    start_date = models.DateTimeField(default=datetime.datetime.now)
    end_date = models.DateTimeField(blank=True, null=True)
    active = models.BooleanField(default=True)
    category = models.CharField(max_length=10, choices=CATEGORY_CHOICES)
    document = models.FileField(upload_to='support_documents')

我想将以下行添加到现有的Document模型中：

network = models.ManyToManyField('selfcare.apps.auth.models.Network')

这是Network类的模型（在selfcare.apps.auth.models中）：

class Network(selfcare.libs.core.utils.SettingHelper, models.Model):
    name = models.CharField(max_length=100, db_index=True)
    code = models.CharField(max_length=20, db_index=True, default='')
    operator = models.ForeignKey(Operator)
    setting = generic.GenericRelation(Setting)

    def __unicode__(self):
        return "%s - %s" % (self.code, self.operator.code)

当我尝试与south进行schemamigration时，出现以下错误消息：

CommandError: One or more models did not validate:
support.document: 'networks' has an m2m relation with model selfcare.apps.auth.models.Network, which has either not been installed or is abstract.

这两个应用程序都在INSTALLED_APP中。这两个应用程序已经由南方管理。