Mocking a nested objects with sinon

I have a test where I'm trying to test the construction of an api call. I am trying to mock the api and am wondering how to access nested objects with sinon.

it('should create the correct fetch method', (done) => {
    let mock = { shop: {get: (...args) => true } };
    let api = sinon.mock(mock);
    let file = sinon.spy();
    let endpoint = 'shop.get';
    let args = {optionA: 'a', optionB: 'b'};

    api.expects(endpoint).withArgs(...args); // throws error

    fetch.call(api, endpoint, file, ...args)
      .then(() => {
        api.verify();
        done();
      });
  });

I'm getting this type error: TypeError: Attempted to wrap object property shop as function