Node express 404 error HTTP status code

I have stripped down my node Express application to the bare minimum. I do not even define routes anymore. When I go to any route, e.g. localhost:3000/sdfsdf, I get the expected output:

{"message":"Not Found","error":{"status":404}}

The HTTP status code of the response however is 200. I expect it to be 404. What am I missing?

var express = require('express');
var path = require('path');

var app = express();
app.use(express.static(path.join(__dirname, 'public')));

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  var err = new Error('Not Found');
  err.status = 404;
  next(err);
});

// development error handler
if (app.get('env') === 'development') {
    app.use(function(err, req, res, next) {
        //res.set('Content-Type', 'application/json');
        res.status(err.status || 500);
        res.status('error').json({
            message: err.message,
            error: err
        });
    });
}
// production error handler
app.use(function(err, req, res, next) {
    res.set('Content-Type', 'application/json');
    res.status(err.status || 500);
    res.status('error').json({
        message: err.message,
        error: {'env': 'prod'}
    });
});

module.exports = app;