Explicitly adding req and res to domain don't propagate error to express middleware

I'm playing around with domains and clusters in node with express and got into this circumstance. I have a cluster that spawns a worker per core, for each worker i create a express-server that uses a domain-per-request strategy to handle errors.

The solution given below is working fine as is but when explicitly adding the request and response objects to the domain the error middleware stops being called. I have no idea why this behaviour is introduced.

Questions:

1. Why is happening?
2. Am I using domains in the correct way?
3. Can i access the current domain in a worker through "process.domain" or do I need to do something different?

Thanks in advance!

My app.js:

var express = require('express')
  , http = require('http')
  , path = require('path')
  , domain = require('domain')
  , cluster = require('cluster')
  , http = require('http')
  , numCPUs = require('os').cpus().length;

if (cluster.isMaster) {

  // fork workers
  for (var i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  // when a worker dies create a new one
  cluster.on('exit', function(worker, code, signal) {
    cluster.fork();
  });

} else {

  var app = express();

  //domains
  app.use(function domainMiddleware(req, res, next) {

    var reqDomain = domain.create();

    res.on('close', function () {
      reqDomain.dispose();
    });

    res.on('finish', function () {
      reqDomain.dispose();
    });

    reqDomain.on('error', function (err) {
      reqDomain.dispose();
      // delegate to express error-middleware
      next(err);
    });

    // Adding the request and response objects to the domain
    // makes the express error-middleware to not being called.
    // reqDomain.add(req);
    // reqDomain.add(res);

    reqDomain.run(next);
  });

  // all environments
  app.set('port', process.env.PORT || 3000);
  app.set('views', __dirname + '/views');
  app.set('view engine', 'jade');
  app.use(express.favicon());
  //app.use(express.logger('dev'));
  app.use(express.bodyParser());
  app.use(express.methodOverride());
  app.use(app.router);
  app.use(express.static(path.join(__dirname, 'public')));

  // for testing which cluster that serves the request
  app.get('/', function(req, res, next) {
    res.json(200, { id: cluster.worker.id });
  });
  app.get('/error', function(req, res, next) {
    var fs = require('fs');
    // intentionally force an error
    fs.readFile('', process.domain.intercept(function(data) { 
      // when using intercept we dont need this line anymore
      //if (err) throw err;
      res.send(data);
    }));
  });

  app.use(function(err, req, res, next) {
    console.log('ERROR MIDDLEWARE', err);
    res.writeHeader(500, {'Content-Type' : "text/html"});
    res.write("<h1>" + err.name + "</h1>");
    res.end("<p>" + err.message + "</p>");
  });

  http.createServer(app).listen(app.get('port'), function(){
    console.log('Express server listening on port ' + app.get('port'));
  });
}