Can't return a value with ZeroRPC NodeJS

Question

I am running in a problem with Javascript and the library ZeroRPC I created a server this way:

Server:

var zerorpc = require('zerorpc');

var server = new zerorpc.Server({
    hello: function(name, reply) {
        reply(null, 'Hello, ' + name);
    }
});

server.bind('tcp://0.0.0.0:4242');

Client

var zerorpc = require('zerorpc');

var client = new zerorpc.Client();
client.connect('tcp://127.0.0.1:4242');

var result = client.invoke('hello', 'RPC', function(error, res, more) {
    //console.log(res); //WORKS ! logs 'Hello, RPC'
    return res; //returns undefined
});

console.log(result); //undefined :(

I don't understand why I cannot return this value from the client.invoke call. Am I missing something out ?