Ember acceptance test not finding root element

I recently introduced acceptance tests in to a ember project that didn't have them after upgrading to 2.13.

I’ve got a very simple test:

test('visiting /', function(assert) {
  visit('/');

  andThen(function() {
    assert.equal(currentURL(), '/');
  });
});

but I’m getting this error:

ember.debug.js:41698 TypeError: Cannot read property 'tagName' of undefined at Class.setup (ember.debug.js:44305) at Class.setupEventDispatcher (ember.debug.js:5084) at Class._bootSync (ember.debug.js:5003)

In my config I have ENV.APP.rootElement = '#ember-testing';

in my app.JS: rootElement: '#applicationContainer'

How do I fix this?