CORS problem with my deno web server

Deno.serve((req) => {
  const webSocketUpgradeRequested = req.headers.get("upgrade") === "websocket";

  if (!webSocketUpgradeRequested) {
    return new Response("Web socket not requested.");
  }

  const { socket, response } = Deno.upgradeWebSocket(req);

  socket.addEventListener("open", () => {
    console.log("Socket open");
  });

  socket.addEventListener("message", (e) => {
    console.log(e.data);
  });

  const header = new Headers();
  header.append("Access-Control-Allow-Origin", "*");

  const res = new Response(response.body, {
    headers: header,
  });

  return res;
});

    onMount(() => {
        const header = new Headers();
        header.append("upgrade", "websocket");

        fetch("http://localhost:8000", {
            headers: header,
        });
    });