Errors on ViewSets with no filterset_class provided

Issue:

This line is causing ViewSets without a filterset_class to break:

django-rest-framework-filters/rest_framework_filters/backends.py

Line 33 in 2fba26e

if issubclass(filterset_class, FilterSet):

It returns:

TypeError: issubclass() arg 1 must be a class

Example ViewSet from my project:

I am using the rest_framework_filters.backends.RestFrameworkFilterBackend globally.

class SavedSearchViewSet(viewsets.ModelViewSet):
    """
    View or edit saved searches.
    """
    queryset = models.SavedSearch.objects.all()
    serializer_class = serializers.SavedSearchSerializer

Thanks!