Http Server upgrade event not being called on Heroku servers

Why 'upgrade' event is not being called on Heroku servers?

var server = new http.Server();

server.on('upgrade', function(req, socket, upgradeHead) {
    // ...
});

Instead, the 'request' event is being called.

I am running the same script in my local machine and with websocket requests the upgrade events is being called.

Is it forbidden?

Thanks.