Runtime.UnhandledPromiseRejection - TypeError: Cannot read properties of undefined (reading 'headers')

I'm getting the above error and I'm stumped as to why.

To explain what I'm doing,

I need to fetch data from one API and then send it to another. This is being used as a background-function job in Netlify and is held on a NextJS site.

I thought it was that I wasn't handling the response correctly so I added them in (as you can see in the code, return res.status(200).json({ success: true, result: result }); for example. But that didn't change anything.

Then I thought maybe it was because I wasn't doing anything with the req in my function, but I'm not sure what to do with that.

It's important to note that this runs locally using netlify-cli with absolutely no problems at all, the whole thing seems to work perfectly, it's only when I deploy it to production that I get this error.

The code:

export default async (req, res) => {
  const coefficientRequest =
    "someURL";

  const headers = new Headers();
  headers.append("Accept", "application/json");
  headers.append("Content-Type", "application/json");

  fetch(coefficientRequest, {
    headers: headers,
    method: "POST",
    body: JSON.stringify({ a: 1, b: 2 }),
  }).then((response) => {
    return response
      .json()
      .then((data) => {
        const myHeaders = new Headers();
        myHeaders.append(
          "someToken",
          "someToken"
        );
        myHeaders.append("Content-Type", "application/json");

        const graphql = JSON.stringify({
          query:
            "someQuery",
          variables: {
            id: "someID",
            metaobject: {
              fields: [{ key: "rates", value: JSON.stringify(data) }],
              handle: "someHandle",
            },
          },
        });
        const requestOptions = {
          method: "POST",
          headers: myHeaders,
          body: graphql,
          redirect: "follow",
        };

        return new Promise(() => {
          fetch(
            "someURL",
            requestOptions
          )
            .then((response) => {
              return response.text();
            })
            .then((result) => {
              return res.status(200).json({ success: true, result: result });
            })
            .catch((error) => {
              return res.status(500).json({ error: error });
            });
        });
      })
      .catch((err) => {
        return res.status(500).json({ error: err });
      });
  });
};

Your help/advice is very much appreciated. Many thanks.

I was never able to solve this problem so I went with another solution.

With PowerAutomate I was able to create a scheduled CRON job running the tasks I needed.