Ember-CLI ember-qunit doesn't seem to wire up model POJO attributes

Testing
1

My Listing model:

import DS from 'ember-data';

export default DS.Model.extend({
  Collections: DS.hasMany('collection', {async: true}), 
  DisplayCompliance: DS.attr(), //POJO
  LastCachedTimestamp: DS.attr('string'),
  ResourceUri: DS.attr('string'),
  StandardFields: DS.attr(), //POJO
  CustomFields: DS.attr(), //POJO
  RentalCalendar: DS.attr() //POJO
});

My util test, which uses the model:

import { test, moduleForModel } from 'ember-qunit';
import getStandardField from 'my/utils/get-standard-field';

moduleForModel('listing', 'getStandardField', {
  needs: ['model:collection']
});

test('can get standard field', function () {
  var listing = this.subject(),
      result = getStandardField(listing, 'TestField');

  Ember.run(function () {
    listing.set('StandardFields', [{'TestField': 'TestValue'}]);
  });

  equal(result, 'TestValue', 'Gets standard field value');
});

When I do a JSON.stringify on the listing object passed into the getStandardFields util function, I get:

{"LastCachedTimestamp":null,"ResourceUri":null,"Collections":[]}

The POJO attributes are missing. Is there a way to wire them up?