Mocha - Testing Promise, `done` never gets called in Promise

I'm trying to test a spy that would be called in the .then block of a promise, but the done in the then block doesn't seem to be executed at all.

Here's what I'm testing (async):

/**
 * Passed down to the LoginForm component to
 * handle form submission.
 */
_submitHandler(data) {
  return function(evt) {
    evt.preventDefault && evt.preventDefault();
    evt.stopPropagation && evt.stopPropagation();

    return request('post', 'auth', data)
      .then((res) => {
        AuthActions.login();
        return res;
      })
  }
}

Here's my test:

describe('when it succeeds', () => {
  it('should login', (done) => {
    sinon.spy(AuthActions, 'login');

    Instance._submitHandler({})({})
      .then((res) => {
        console.log('Called!!!');
        expect(AuthActions.login.called).to.equal(true);
        AuthActions.login.restore();
        done();
      }, done);
  });
});

I'm using Karma to run my tests; Chai and Sinon.