bcrypt Error: data and hash arguments required

I am getting a bcrypt error stating that data and hash arguments are required, referencing line #44 in my routes.js file. From what I can tell, I am passing that information: the first parameter to bcrypt.compare is the user entered password, and the second is the hashed password retrieved from the db. What am I doing wrong?

bcrypt.compare(req.params.password, user.password, function...

routes.js

'use strict'

var express = require('express');
var router = express.Router();
var User = require('../app/models/user');
//password hashing
var bcrypt = require('bcrypt');

var count = 0;

router.use(function(req, res, next) {
    count++;
    console.log('API hit count = %s', count);
    next();
});

// /users post(create new user) get(specific user)
router.route('/users')
    .post(function(req,res) {
        var user = new User();
        user.username = req.body.username;
        user.password = bcrypt.hashSync(req.body.password, 10);

        //save the user and checkfor errors
        user.save(function(err) {
            if (err) {
                res.send(err);
            } else {
                res.json({message: "User created!"});
            }    
        });

    })

router.route('/users/:username')
    .get(function(req, res) {
        var query = {
            username: req.params.username,
        };
        User.findOne(query, function(err, user) {
            if (err) { 
                res.send(err);
            } else {
                bcrypt.compare(req.params.password, user.password, function(err, res) {
                    if(err) {
                        console.log('Comparison error: ', err);
                    }
                })
                res.json(user);
            }
        });
    })

Answer

bcrypt.compare takes 3 parameters; passwordToCheck, passwordHash, and a callback, respectively. (Check the documentation for examples)

This error means one or both of the first 2 parameters are either null or undefined. Therefore, make sure both of them are passed correctly. (Not as null or undefined)