程序出现未知错误

F:\nodejs\myapp\nodebeginner>node index.js
Server has started.
Request  /start received
About to route a request for /start
Request handler 'start' was called.

F:\nodejs\myapp\nodebeginner\requestHandlers.js:18
response.writeHead(200, {"Content-Type": "text/html"});
^
TypeError: Cannot call method 'writeHead' of undefined
at Object.start (F:\nodejs\myapp\nodebeginner\requestHandlers.js:18:12)
at route (F:\nodejs\myapp\nodebeginner\router.js:4:26)
at Server.onRequest (F:\nodejs\myapp\nodebeginner\server.js:10:17)
at Server.emit (events.js:98:17)
at HTTPParser.parser.onIncoming (http.js:2112:12)
at HTTPParser.parserOnHeadersComplete [as onHeadersComplete]
(http.js:121:23)
at Socket.socket.ondata (http.js:1970:22)
at TCP.onread (net.js:527:27)