Chai deep.equal failing on comparison of {} to promise instance

Trying to understand why deep.equal is still passing when the two objects are very different:

it.only('fetches a footer', () => {
  const fetchedAction = { type: FETCH_FOOTER, payload: {} };
  mockGet(host, 'footer.json', null, null);

  const action = fetchFooter();
  expect(action).to.deep.equal(fetchedAction);
});

So here are the values being compared for action vs. fetchedAction:

Notice how for the payload property they're not the same type of object. Then how come my expect isn't failing if that's the case?

Related (yea I read this but still not sure why simply deep.equal wouldn't fail here): http://ift.tt/2uIw92B