Error: Promised response from onMessage listener went out of scope

import type { PlasmoMessaging } from "@plasmohq/messaging";

const handler: PlasmoMessaging.MessageHandler = async (req, res) => {
  try {
    console.log("background: translate")
    const body = { input_text: req.body.input_text};
    const res2 = await fetch('http://localhost:3000/translate', {
      method: 'POST',    
      headers: { 'Content-Type': 'application/json'},
      body: JSON.stringify(body)
    });
    const {output_text} = await res2.json();
    console.log(output_text);
    res.send({...body, output_text: output_text});
  } catch (err) {
    console.log(err);
    res.send({ error: err.message ?? "Unknown error" });
  }
};

export default handler;