Socket.io in a module - node.js

Question

I'm trying to create a chat module using socket.io. Ideally I would like to pass the port number like this:
var anotherChatApp = require("anotherChatApp")(1337);
the problem I'm having is finding the object after I've set the listen() method.
var io = require("socket.io");
function anotherChatApp(port){
io.listen(port);
}
then I don't know where it goes, because I try to access the io object:
io.sockets.on('connection', function (socket) {
//dosomething
}
and I get runtime errors:
Cannot call method 'on' of undefined
I was loving node until I started building a module.
EDIT:
var io = require("socket.io");
function MacroChat(portNo){
io = require("socket.io").listen(portNo);
}
module.exports = MacroChat;
//here is where the errors come in
io.sockets.on('connection', function (socket){
});
And the error:
TypeError: Cannot read property 'sockets' of undefined
at Object.<anonymous> (D:\wamp\www\nodejitsu\macrochat\node_modules\MacroChat\lib\macrochat.js:10:3)
at Module._compile (module.js:456:26)
at Object.Module._extensions..js (module.js:474:10)
at Module.load (module.js:356:32)
at Function.Module._load (module.js:312:12)
at Module.require (module.js:364:17)
at require (module.js:380:17)
at Object.<anonymous> (D:\wamp\www\nodejitsu\macrochat\node_modules\MacroChat\index.js:5:18)
at Module._compile (module.js:456:26)
at Object.Module._extensions..js (module.js:474:10)