Ember component integration test: inFlight error on model save

I am trying to test a component handling newsletter-subscriptions. I want to test whether the submitForm function saved successfully by setting a class of a hidden input accordingly after the save happened or did not happend so that I can assert this in an integration test.

Here is my code:

newsletter.subscription.js:

import Ember from 'ember';

export default Ember.Component.extend({
  store: Ember.inject.service(),
  displayNotifications: Ember.inject.service(),

  didInsertElement() {
    this.set('model', this.get('store').createRecord('subscription'));
  },
  actions: {
    submitForm() {
      this.get('model').save().then(
        (json) => {
          this.get('displayNotifications').success({
            key: 'subscription.success',
          });
          this.set('model', this.get('store').createRecord('subscription'));
          this.set('subscribed', true);
        },
        (reason) => {
          this.set('subscribed', 'error');
        });
     }
   }
});

Here is the code of the newsletter-subscription.hbs:

<div class="ui container row newsletter-form">
  <div class="ui inverted segment">
    <form class="ui form" >
      <div class="ui stackable grid">
        <div class="eleven wide right aligned column">
          
        </div>
        <div class="five wide inverted left aligned column">
          <button type="submit" class="ui fluid secondary button ">
            
          </button>
        </div>
        <input type='hidden' id="debug" class="" />
      </div>
    </form>
  </div>
</div>

And here is my test: newsletter-subscription-test.js

 moduleForComponent('newsletter-subscription', 'Integration | Component | newsletter-subscription', {
   integration: true
 });


 test('newsletter-subscription: submitting newsletter-subscription sets subscribed to true', function(assert){

    this.render(hbs``);

    assert.equal(this.$('#debug').hasClass("notSubscribed"), true, 'before click, debug element has class notSubscribed');

    this.$('input[name=subscription-email]').val("test@example.com");
    this.$('button[type=submit]').click();

    return wait().then( () => {
      assert.equal(this.$('#debug').hasClass("subscribed"), true, "after callback of click ran, debug element has class subscribed");
    });

 });

The first assertion works. But then I keep getting an error:

✘ Assertion Failed: You can only unload a record which is not inFlight.

From breakpoint analysis, I found that this happens when this.get('model').save() is called. My tries to resolve the issue remained ineffective so far (i.e. mocking server response using mirage). Is there an issue with the Ember run loop in this context that I somehow have to take care of or something different?

Thank you for your input!