angular.js Assert error throw async

Question

I'm testing an angular.js app using karma with sinon-chai. I need to test a code like this:

var sync = function() {
  async.then(function() {
    // success
  }, function() {
    throw Error('foo');
  });
}

sync is the function I want to test. async is a function returning a $q promise I want to test it fails throwing an error.

My ideal test suit:

describe('unit', function() {
  it('should throw an error', function() {
    expect(function() {
      return sync();
    }).to.throw(Error, 'foo');
  })
});

But, it fails at afterEach() because the error is thrown on $digest.

How can I achieve this test?

Thank you very much!