使用formidable模块报错，求大神解答

今天在做一个图片上传的功能，在用formidable的时候遇到一个问题，查了很多资料，但是渣渣的我还是找不到原因，求大神解答解答。

下面是我的报错的提示：

而我的代码如下：

入口文件 blog.js

   var express=require('express'),
   	mongoose=require('mongoose'),
   	mongoStore=require('connect-mongo')(express),
   	blog=module.export=express(),
   	ejs=require('ejs'),
   	dbUrl='mongodb://localhost/myblog',	
   	SessionStore=require('session-mongoose')(express),
   	db;	
   mongoose.connect(dbUrl);
   db=mongoose.connection;	
   var store=new SessionStore({
   	url:dbUrl,
   	interval:120000
   })	
   blog.configure(function(){
   	blog.set('views',__dirname+'/blog/views');
   	blog.engine('.html',ejs.__express);
   	blog.set('view engine','.html');
   	blog.set("view options",{
   		layout:false
   	})
   	blog.use(express.bodyParser());
   	blog.use(express.methodOverride());
   	blog.use(express.cookieParser());
   	blog.use(express.cookieSession({
   		key:'user',
   		secret:'myblog',
   		store:store,
   		cookie:{maxAge:1200000}
   	}));
   	blog.use(express.static(__dirname+'/public'));
   	
   });
   db.on('error',function(err){
   	console.log("link to database error!");
   })	
   db.once('open',function(){
   	require('./config/route')(blog);
   })	
   blog.listen(4000)

路由文件 route.js

   		  var coverApi=require('../blog/controllers/cover');	  
   		module.exports=function(blog){		  
   		  blog.use(function(req,res,next){
   			  var _user=req.session.user;
   			  blog.locals.user=_user;
   			  next();
   		  })			  
   		 blog.post('/manage/addCover.html',coverApi.addCover);	  
   	  }

控制器文件 cover.js

   	 var fs=require('fs'),
   	path=require('path'),
   	formidable=require('formidable'),
   	Article=require('../models/article'),
   	coverApi={},
   	util = require('util');	
   module.exports=coverApi;	
   coverApi.addCover=function(req,res){ 
   
   	var form = new formidable.IncomingForm({uploadDir:'./sources/'});
   	form.parse(req, function(err, fields, files) {
   		if(err) console.log(err);//这里报错了
   	  res.writeHead(200, {'content-type': 'text/plain'});
   	  res.write('received upload:\n\n');
   	  res.end(util.inspect({fields: fields, files: files}));
   	});
   	
   }	
   coverApi.show=function(req,res){
   	var id=req.params.id;
   	res.render('cover',{
   		id:id
   	});	
   }

视图文件 cover.html

	<div class="container">
		<h1>添加图片</h1>
		<form action="/manage/addCover.html" method='post' enctype="multipart/form-data">
			<div class="form-group">
				<input type="text" class='hidden' value="<%= id%>" name='id'>
				<label for="">图片</label>
				<input type="file"  name='cover'>
				<button type='submit' class='btn btn-lg btn-primary'>add</button>
			</div>
		</form>
	</div>

package.json 文件

项目目录结构

现在那个问题还是解决不了，求大神帮忙解答