Whoops. Need to be good and use six.

    def resolve_related_fields(self):

        if len(self.from_fields) < 1 or len(self.from_fields) != len(self.to_fields):

            raise ValueError('Foreign Object from and to fields must be the same non-zero length')

        if isinstance(self.rel.to, basestring):

        if isinstance(self.rel.to, six.string_types):

            raise ValueError('Related model %r cannot been resolved' % self.rel.to)

        related_fields = []

        for index in range(len(self.from_fields)):