Using fetch with wss:// Upgrade: websocket fails

export default {
    async fetch() {
        // URL of your WebSocket server
        const url =
            'wss://REDACTED';

        // Custom headers
        const headers = {
      Upgrade: 'websocket',
            Cookie:
                'REDACTED',
        };

    let resp = await fetch(url, {
      headers
    });
  
  let ws = resp.webSocket;
  if (!ws) {
    throw new Error("server didn't accept WebSocket");
  }

  // Call accept() to indicate that you'll be handling the socket here
  // in JavaScript, as opposed to returning it on to a client.
  ws.accept();

  let text = ''
  
  ws.addEventListener('message', msg => {
    text = JSON.parse(msg.data);
  });

  await new Promise((resolve) => setTimeout(resolve, 5000));        
  
  return new Response(text);
  
    },
};