how to fix?

[vinnitu]

$ node rpcdemo.js
running

http:432
throw new Error("You have to call writeHead() before write()");
^
Error: You have to call writeHead() before write()
at ServerResponse.write (http:432:11)
at ServerResponse.end (http:516:16)
at Server. (/var/home/sklyar/node.js/andris9-node-jsonrpc-a46b7ca/rpcdemo.js:12:18)
at Server.emit (events:27:15)
at HTTPParser.onIncoming (http:831:10)
at HTTPParser.onHeadersComplete (http:87:31)
at Stream.ondata (http:763:22)
at IOWatcher.callback (net:494:29)
at node.js:768:9