Django REST Framework - Could not resolve URL for hyperlinked relationship using view name "group-detail"

I'm setting up something similar to the Django Rest Framework quickstart example, but with custom django-authtools User model.

api.py:

router = routers.DefaultRouter()
router.register(r'users', views.UserViewSet)
router.register(r'groups', views.GroupViewSet)

urlpatterns = [
    url(r'^', include(router.urls)),
    url(r'^api-auth/', include('rest_framework.urls', namespace='rest_framework')),
]

serializers.py:

class GroupSerializer(serializers.HyperlinkedModelSerializer):

    class Meta:

        model = Group
        fields = ('url', 'name')


class UserSerializer(serializers.HyperlinkedModelSerializer):

    class Meta:
        model = User
        fields = ('name', 'email', 'groups')

views.py:

class UserViewSet(viewsets.ModelViewSet):
    """
    API endpoint that allows users to be viewed or edited.
    """
    queryset = User.objects.all()
    serializer_class = UserSerializer


class GroupViewSet(viewsets.ModelViewSet):
    """
    API endpoint that allows users to be viewed or edited.
    """
    queryset = Group.objects.all()
    serializer_class = GroupSerializer

I'm running Django 1.8.5. Is there something extra I have to configure to work with the custom user model, or I have I just missed something silly?