Error when using promises to retrieve object from mongo db

Folks, I am trying to wrap the following with promises, and return a Mongo document.

Controller's catch is getting error: 'TypeError: Cannot call method \'apply\' of undefined'

What is the proper way to return the Object from the db to the controller?

Controller:

Q.fcall(Users.fetchId(authId)).then(function userVerification(userObject) {
    console.log ('got back',userObject);
    responses.Unauthorized('Unauthorized ID', res);
}).catch(function handleError(err) {
    responses.InternalServerError(''+err, res);
});

Users Model:

Users.prototype.fetchId = function fetchId(authId) {
    return this.mongodb.fetchId(authId);
};

Mongo:

MongoDatabase.prototype.fetchId = function fetchId(id) {
    var result = {}
    return this.authDB.query('users', function(collection) {
        return collection.findOne({_id:id}, function (err, doc) {
            if (!_.isEmpty(doc)) {
                var result = doc;
            }
            console.log ('mongo',result);
            return result;
        });
    });
};