Why am I getting this error on Heroku but not locally?

Question

TypeErrorserver/models/User.js:143 Object # has no method 'pbkdf2Sync'

The app works fine locally.

var crypto = require('crypto');

encryptPassword: function(password) {
    if (!password || !this.salt) return '';
    var salt = new Buffer(this.salt, 'base64');
    return crypto.pbkdf2Sync(password, salt, 10000, 64).toString('base64');
}