social_django.models.UserSocialAuth.DoesNotExist in unit test

I have both model and social backends configured in settings.py:

AUTHENTICATION_BACKENDS = (
    'django.contrib.auth.backends.ModelBackend',
    'social_core.backends.mediawiki.MediaWiki',
)

When I run this test:

from django.test import TestCase, Client
from django.contrib.auth.models import User


class UserActivitiesViewTest(TestCase):
    def test_mainspace_title_contains_colon(self):
        user_fred = User.objects.create_user('Fred', 'fred@example.com', 'password')
        client = Client()
        client.force_login(user_fred, backend='django.contrib.auth.backends.ModelBackend')
        response = client.get('/spi/spi-user-activities/Foo', {'count': 10, 'main': 1}, follow=True)

The view that's being tested is:

class UserActivitiesView(LoginRequiredMixin, View):
    def get(self, request, user_name):

      .....

I get a social_django.models.UserSocialAuth.DoesNotExist exception in the client.get() call.  Why is it doing any kind of query on UserSocialAuth if I'm telling force_login() to use ModelBackend?

I'm running django 2.2, python 3.7