socket.io errors when client connects

I have socket.io running successfully on my node.js installation.

info  - socket.io started

The console shows it running ok, but as soon as the client (browser) connects:

socket = io.connect('<my host>:8000');

The console is kicking out an error:

crypto.js:123
  return new Hash(hash);
         ^
TypeError: undefined is not a function
    at Object.createHash (crypto.js:123:10)
    at WebSocket.onSocketConnect (/node_modules/socket.io/lib/transports/websocket/hybi-16.js:120:23)
    at WebSocket.handleRequest (/node_modules/socket.io/lib/transport.js:71:10)
    at WebSocket.Transport (/node_modules/socket.io/lib/transport.js:31:8)
    at new WebSocket (/node_modules/socket.io/lib/transports/websocket/hybi-16.js:59:13)
    at new WebSocket (/node_modules/socket.io/lib/transports/websocket.js:31:17)
    at Manager.handleClient (/node_modules/socket.io/lib/manager.js:661:19)
    at Manager.handleUpgrade (/node_modules/socket.io/lib/manager.js:618:8)
    at Server.<anonymous> (/node_modules/socket.io/lib/manager.js:123:10)
    at Server.emit (events.js:88:20)

can anyone understand what this might mean?

My code has been working fine on my local machine, its only when moving it to my production server does this error happen.