Python test assertContains – AttributeError: 'Response' object has no attribute 'streaming'

I am testing a html response object with assertContains:

self.assertContains(response, "", status_code=200)

and am getting the error:

AttributeError: 'Response' object has no attribute 'streaming'

The response object comes with a 200 status code and seems well-formed (from Mailchimp API). Why should this fail if there is no streaming attribute?