Socket.IO send() not working

I have a weird issue. I'm writing a simple Socket.IO echo server as a test, but I can't get the send() method to work. On the server I'm using this piece of code:

var io = require('socket.io').listen(3000); 
var clients = {};

io.sockets.on('connection', function (socket) {     
        socket.send('test');

        socket.on('addclient', function(id) {
            socket.id = id;
            clients[id] = socket;
            console.log('New client connected: '+ id);
        });

        socket.on('echomessage', function(message) {
            console.log('Sending echo message to client '+ socket.id);
            socket.send(message);
        });

        socket.on('disconnect', function() {
            console.log('Client '+ socket.id + ' disconnected');
            delete clients[socket.id];
        });
});

The send('test') is working perfectly, but when I emit the echomessage event, I don't receive the message. No error messages at all on the server or client side.

So, on the clientside I do this:

// Connect with the server (works, connection established)

// works too, I see it on the server
sock.emit('addclient', 1); 

// I see 'Sending echo message to client 1' on the server
sock.emit('echomessage', 'Echo this please');

But I don't receive the message at all.

I have absolutely no clue of what I'm doing wrong. All help is appreciated!