Stub ES6 class with sinon.js

I have a controller class which instantiates a model class and I want to test that the controller uses the correct parameters when it instantiates the model. I have found that stubbing methods on a class with sinon is no problem, but if I need to stub the constructor, I can't get it to work.

This is my controller:

import settings from '../../config/settings';
import model from '../models/Form';

let content;

export default class Form {

  constructor (app) {
    content = new model(app.settings.content);
  }
}

And this is the test (so far)

import {assert} from 'chai';
import sinon from 'sinon';
import fs from 'fs';
import settings from '../../../config/settings';
import model from '../../../lib/models/Form';
import controller from '../../../lib/controllers/Form';

let mocks;

describe('Form controller', () => {

  beforeEach((done) => {
    mocks = {};
    mocks.model = sinon.createStubInstance(model);
    done();
  });

  afterEach((done) => {
    mocks = null;
    done();
  });

  describe('New Forms controller', () => {

    beforeEach((done) => {
      mocks.app = {
        settings: {
          content: '/content/path/',
          views: '/views/path/'
        }
      };
      mocks.controller = new controller(mocks.app);
      done();
    });

    it('Instantiates a model', (done) => {
      assert.isTrue(mocks.model.calledWith(mocks.app.settings.content));
      done();
    });
  });
});

I run the tests with this command:

npm run test-unit
//which equates to
"test-unit": "BABEL_DISABLE_CACHE=1 ./node_modules/.bin/mocha --recursive --check-leaks --reporter spec --bail --compilers js:babel/register ./test/unit"

The model class is built with the same pattern as the controller (e.g. export default class, constructor, etc). The problem is that in the controller constructor the model is not a stub but just the class itself.

Any suggestions on how to do this, or even whether I need to be testing this are more than welcome.