Heroku Error during websocket handshake 503

I'm trying to coect to websockets in heroku but it's saying Error during websocket handshake: Unexpected response code: 503. The error in Dev Tools is 'Service unavailable'.

Server code

var wss = new WebSocketServer({server: app,  port:5001});

Client code(I am replacing the port to 5001 as well)

  var host = location.origin
          .replace(/^http/, 'ws')
          .replace('5000','5001');


var ws = new WebSocket(host);

I've did the same in development and I managed to coect. Any help to troubleshoot? Thanks.