Unit testing a promise where the input params are invalid

I'm trying unit test a promise ["q": "^1.1.2"]; the following code works when the error is not being thrown. I need to validate the expected behaviour that the error is handled correctly.

Code:

 Service.prototype.getItem = function (itemId) {
    var deferred = Promise.defer();

    // Ensure
    if (typeof itemId !== 'string' || itemId === '') {
        return deferred.reject(new Error('itemId is a required field'));
    }
  .... 
    // do some logic at return item
    return deferred.resolve(item);
  ....

  return deferred.promise;

Since the method hasnt returned the promise, I cant test it for error handling! Is there a better way for the service to handle the promise but to return an error if one of the params are wrong.

Unit Test:

 it('Call Service - should throw an error', function (done) {
        Service.getItem('')
            .then(function (item) {})
            .catch(function (err) {
                expect(err).not.to.be.empty();
                done()
            }).done();
    });