Websocket subprotocol results in Error: The script will never generate a response

const ws = new WebSocket(wsUrl, "potato");

app.get(
  "/ws",
  upgradeWebSocket((c) => {
    const rawProtocols = c.req.header("sec-websocket-protocol");
    console.log(rawProtocols);

    return {
      onMessage: (event) => {
        console.log(event.data);
      },
    };
  })
);