Sending confirmations with socket.io says my function is undefined

I am trying to use confirmations with socket.io as defined in http://socket.io/#how-to-use "Sending and receiving data (confirmations)".

My client code looks like

sio.emit('ferret', 'tobi', function (data) {
  console.log(data);
});

And my server code:

socket.on('ferret', function (name, fn) {
  fn('woot');
});

But I get an error message that breaks my node server:

fn('woot');
23:13:48 web.1  |         ^
23:13:48 web.1  | TypeError: undefined is not a function

Which I find strange, because if I add:

console.log(name);
console.log(fn);

the name will be written by Tobi, but for what reason fn registers undefined.

Any info would be awesome!