Redis MULTI command on NodeJS Heroku

I have been trying to use the MULTI command with redis on Heroku, it works well a couple of times but soon or later it stops working and returns 'QUEUED'. It always happens when I use MULTI.

redis.multi()
    .hgetall('project:' + url)
    .hgetall('code:' + url)
    .exec(function (err, results) {
        //...
    });

It's working fine on my local machine but it crashes on Heroku all the time.

Why does 'QUEUED' means here? Has anyone had this issue on Heroku?