Not getting posts object

Object.keys(posts).map(function(value){return posts[value]})
TypeError: Cannot convert undefined or null to object
No Properties
Runtime.getProperties failed.
Object #null# not found

'use strict';

var express = require('express'),
    posts = require('./mock/posts.json');

var app = express(); 

app.use('/static', express.static(__dirname + '/public'))

app.set('view engine', 'jade');
app.set('views', __dirname + '/templates')

app.get('/', function(req, res){
    res.render('index')
});

app.get('/blog/:title?', function(req, res){
    var title = req.params.title;
    if (title === undefined){
        res.status(503);
        res.render('blog')
        } else {
        var post = posts[title] || {};
        res.render('post', { post: post}); 
        }       
});

app.listen(3000, function(){
    console.log("the frontend server is running on port 3000");
});