TypeError：标头名称必须是nodejs中的有效http标记

Question

这是我的代码

var http = require('http');
var hostname = 'localhost';
var port = 3000;

var server = http.createServer(function (req,res) {
    console.log(req.headers);
    res.writeHead(200,{'content-type':'text/html'});
    res.end('<h1>This is hello word</h1>');
});

server.listen(port,hostname,function () {
    console.log(`server running at http://${hostname}:${port}/`);
});

我遇到错误，我使用的节点版本是v4.4.7