How to avoid a 500 error for express when there's no Accept-Language header in the request while I have some logic based on that header?

I am working on a express(4.16.3) web server.

The page would do some special things according to the browser's language setting.

However, if the browser (for instance, some server bots) does NOT have an Accept-Language header, it would bring a 500 error.

Here is the original version:

//index.js - original - would return a 500 Internal Server Error

utils.catchAsyncErrors(async (req, res) => {
    let lang = req.params.lang
    if (!lang) {
      const langs = accepts(req).languages()
      try {
        lang = langs[0].split('-')[0].toLowerCase()
      } catch (err) {}
    }

Here is the modified version - I use try...catch block but it fails to bypass the error.

//index.js - original - would still return a 500 Internal Server Error

utils.catchAsyncErrors(async (req, res) => {
  let lang = req.params.lang
  if (!lang) {
    var langs;
    try {
      langs = accepts(req).languages();
    } catch (err) {
      langs = 'en';
    }
    try {
      lang = langs[0].split('-')[0].toLowerCase()
    } catch (err) {
      lang = 'en';
    }
  }

If I comment out the entire logic about the language header check, it works.

//index.js - original - would work fine

utils.catchAsyncErrors(async (req, res) => {
  let lang = req.params.lang
  if (!lang) {
    /*
    var langs;
    try {
      langs = accepts(req).languages();
    } catch (err) {
      langs = 'en';
    }
    // const langs = accepts(req).languages()
    try {
      lang = langs[0].split('-')[0].toLowerCase()
    } catch (err) {
      lang = 'en';
    }
    */
    lang = 'en';
  }

Just FYI, the other function mentioned in my code.

//utils.js - the catchAsyncErrors definition

exports.catchAsyncErrors = function (fn) {
  return (req, res, next) => {
    const routePromise = fn(req, res, next)
    if (routePromise.catch) {
      routePromise.catch(err => next(err))
    }
  }
}

What should I do to make the server work fine while it can both:

• Check the Accept-Language header.
• Not failing when there's no Accept-Language header presented.