Error running with express and nextjs - TypeError [ERR_INVALID_ARG_TYPE]: The "chunk" argument must be of type string or an instance of Buffer. Received an instance of Promise

I’m using express with nextjs, but pulled the express app out. Code look llike so

// server.js 
const next = require('next');
const { initializeExpressWithNext } = require('./src/server/app');
const dev = process.env.NODE_ENV !== 'production';
const nextApplication = next({ dev });
const handle = nextApplication.getRequestHandler();

nextApplication.prepare().then(() => {
  const app = initializeExpressWithNext(handle);

  app.listen(3000, (err) => {
    if (err) throw err;
    console.log('Ready on http://localhost:3000');
  });
});


//app.js
const express = require('express');
const cors = require('cors');
const apiMetrics = require('prometheus-api-metrics');

const questionnaireRoutes = require('./routes/questionnaire');
const recaptchaRoutes = require('./routes/recaptcha');

/**
 * @function initializeExpressWithNext
 * @description to run this with nextjs, we need to pass in a callbackhandler
 * for all routes that are not defined by express, we do this by wrapping express
 * in a function to initialize the callback and runtime
 * @param {function} callbackHandler - nextjs server callbackhandler
 * @return {*} express application
 */
function initializeExpressWithNext(callbackHandler) {
  const app = express();
  app.use(apiMetrics());

  app.get('/healthcheck', cors(), (_, res) => {
    res.status(200).send();
  });

  app.use('/', questionnaireRoutes);
  app.use('/', recaptchaRoutes);

  app.get('*', (req, res) => {
    return callbackHandler(req, res);
  });

  return app;
}

module.exports = { initializeExpressWithNext };

The app starts fine, but when i navigate to /metrics or metrics.json, I get the following error

TypeError [ERR_INVALID_ARG_TYPE]: The "chunk" argument must be of type string or an instance of Buffer. Received an instance of Promise
    at ServerResponse.end (_http_outgoing.js:784:13)
    at ExpressMiddleware.middleware (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/prometheus-api-metrics/src/express-middleware.js:99:24)
    at Layer.handle [as handle_request] (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/router/index.js:317:13)
    at /Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/router/index.js:335:12)
    at next (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/router/index.js:275:10)
    at expressInit (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/middleware/init.js:40:5)
    at Layer.handle [as handle_request] (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/router/index.js:317:13)
    at /Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/router/index.js:335:12)
    at next (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/router/index.js:275:10)
    at query (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/middleware/query.js:45:5)
    at Layer.handle [as handle_request] (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/Users/nathanhall/bwell/medstar-squeeze-page/node_modules/express/lib/router/index.js:317:13)
    ```
    
    I'll try to see if I can figure it out and make. PR but I'm on a tight timeline