Django Rest Framework - “detalhe”: “Não encontrado.”

Olá, ao fazer esta solicitação: groups / 25010a31-fc5b-47c8-9c5c-d740e5743f52 / members / 4 / - recebo "detail": "Not found"

No entanto, se você olhar no conjunto de consultas, imprimi a instância Groupmember e isso acaba imprimindo essa instância específica com tanta clareza que existe?

Visão:

class MemberDetail(mixins.RetrieveModelMixin,
                   mixins.DestroyModelMixin,
                   mixins.UpdateModelMixin,
                   generics.GenericAPIView):
    serializer_class = GroupMembersSerializer
    lookup_field = "user_id"
    lookup_url_kwarg = "uuid"

    def get_queryset(self):
        group = self.kwargs["uuid"]
        user_id = self.kwargs["user_id"]
        print GroupMember.objects.get(group = group, user_id = user_id)
        return GroupMember.objects.get(group = group, user_id = user_id)

    def get(self, request, *args, **kwargs):
        return self.retrieve(self, request, *args, **kwargs)

    def put(self, request, *args, **kwargs):
        return self.update(self, request, *args, **kwargs)

    def delete(self, request, *args, **kwargs):
        return self.destroy(self, request, *args, **kwargs)

URLs:

 urlpatterns = [

    url(r'^$', views.GroupList.as_view()),
    url(r'^(?P<uuid>[^/]+)/$', views.GroupDetail.as_view()),
    url(r'^(?P<uuid>[^/]+)/members/$', views.MemberList.as_view()),
    url(r'^(?P<uuid>[^/]+)/members/(?P<user_id>[0-9]+)/$', views.MemberDetail.as_view()),

] 

Qualquer ajuda?