Test method in CBV in Django

I have the following CBV

class Index(TemplateView):
    template_name="index.html"

    def custom_method(self):
        # do some stuff here
        print("Test")

How can I test this method in a TestCase? I've tried the following but it's not working:

def test_custom_method(self):
    response = self.client.get(reverse("index"))
    view_instance = Index.as_view()
    view_instance(response.wsgi_request)
    view_instance.custom_method()