Using rails url_helper to specify subdomain causes failing tests

I’m getting this error on every test that involves a url_helper:
ArgumentError: Missing host to link to! Please provide the :host parameter, set default_url_options[:host], or set :only_path to true

Tests are all green locally, but when I run from the circleci container I get the above error. I suspect it has something to do with me misunderstanding how to properly set the host on circleci’s test machine and/or the circleci ignoring my explicitly setting default_url_options in the app’s test.rb.

Should I be setting a different host in the circle.yml file? If so, what? Should I then also set my default host for all tests to that?