MQTT connection options

I am running v1.87 on an esp8266. I am able to subscribe to an MQTT broker running on my local network. I can publish and subscribe successfully.

I am attempting to publish from one esp8266 and subscribe from another. I believe that both boards use the same default client_id, because when I connect the second one, the first is disconnected.

I would expect to be able to work around this by setting a client_id in the MQTT connection options, as described here: http://www.espruino.com/MQTT

My code looks like this:

var server = "192.168.2.10";

        mqtt = require("MQTT").create(server);
        mqtt.connect({
            client_id: 'webmos02'
        });

        mqtt.on('connected', function() {
            mqtt.publish('led', 'webmos02 start');
            mqtt.subscribe("led");
            print('subscribed to led');
        });

        mqtt.on('publish', function(pub) {
            print("topic: " + pub.topic);
            print("message: " + pub.message);
        });

I get these errors:

Uncaught Error: Function "write" not found!
 at line 1 col 35
....log("Client connected");a.write(b.mqttConnect(b.client_id))...
                              ^
in function "e" called from line 3 col 353
..."close")});b.client=a};a?e():a=require("net").connect({host:...
                              ^
in function "connect" called from line 1 col 131
...nect({client_id:'webmos02'}),mqtt.on('connected',function(){...
                              ^
in function called from system

I have tried putting in other things in the connection options, like _protocolname: "MQTT" and get the same result. I get errors regardless of what I put in as a connection option.

If I leave out the connection option I am able to connect and both publish and subscribe.

1. Am I correct in assuming that two different esp8266s without a client_id set will use the same client_id and thus collide?
   
2. How do I set a client_id?
   
3. Is there possible another way to resolve this issue?