passport-twitter returns "Failed to load resource: the server responded with a status of 500 (Internal Server Error)"

Question

I am stuck trying to solve the problem, but ended up with here. This is the erroneous code repo.

I got an error saying:

Failed to load resource: the server responded with a status of 500(Internal Server Error)`

and for such a code snippet, it always redirected to the failure URL (/routes/auth.js):

.get(passport.authenticate('twitter',
    {
        successRedirect: '/users/',
        failureRedirect: '/error/'
    })
);

Based on the current commit, I can successfully get the token from Twitter API. The callback URL is as such:

http://localhost:3000/auth/twitter/callback?oauth_token=(token)&oauth_verifier=(token)

Questions:

1. What went wrong?
2. If trying to debug the error, how could I construct the error-first callback (I did NOT even know where and how to output debug logs)?