node.js mongojs findOne callback returning error as null

Presently being driven up the wall by this error.

I am running a node.js app with the mongojs wrapper for mongodb. I Started mongod on the default port, then ran

var db = require('mongojs').connect('localhost:27017/my_db');
var users = db.collection('users');    
users.findOne({'fb_id' : fbUserMetadata.id}, function(err, user) {
                console.log(err);
                console.log(user);
                debugger;

    });

however err and user are both 'null'. To my knowledge, err should be populated with some kind of data even if it doesn't find anything.

How do I get the callback function to work properly? Pardon the newbie question.