Unhandled promise rejection in Node server, only on Heroku, works locally

I have a node app that serves a react app as well as makes requests to Google Cloud Storage. The App works perfectly locally, but after I've deployed it to Heroku I get the following error whenever I make requests to any of my endpoints:

2017-05-26T21:53:34.426234+00:00 app[web.1]: app.post /upload_url Endpoint
2017-05-26T21:53:34.484393+00:00 app[web.1]: (node:34) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): RangeError: Invalid status code: 0

The first line is a console log to check if the endpoint is reached, the second line is the error. Here's the code for the `/upload_url' endpoint:

app.post('/upload_url', (req, res) => {
  console.log("app.post /upload_url Endpoint");
  var originalFileName = req.body["originalFileName"];
  var buf = crypto.randomBytes(16);
  var uniqueFilename = buf.toString('hex') + '_' + originalFileName;
  var file = bucket.file(uniqueFilename);

  file.createResumableUpload(function (err, uri) {
    if (!err) {
      var json = JSON.stringify({
        uri: uri,
        uniqueFilename: uniqueFilename,
        originalFileName: originalFileName
      }); 
      res.writeHead(200)
      res.end(json);
    } else {
      res.writeHead(err.code);
      res.end();
    }
  });
});

This endpoint is called by the react front end with the following fetch call:

function upload(file) {
  var data = new FormData();
  data.append('file', file);
  return fetch(`upload_url`,{
    method: 'POST',
    headers: new Headers({
      "Content-Type": "application/json",
    }),
    body: JSON.stringify({
        originalFileName: file.name
    })
   });
  }
}

Again, this works fine in development but not after deploying to Heroku. I've tried Heroku's suggestion of adding concurrency to the app (detailed here) without any luck. Any thoughts or suggestions on how to solve this problem would be very much appreciated.

via quicklikerabbit