Koa's `ctx.status` not getting sent to client

Here is my simple route:

router.post('/getFile', async (ctx) => {
  const fileName = `${ctx.request.body.file}.pdf`;
  const file = fs.createReadStream(fileName); // This file might not exist.

  file.on('error', (err) => {
    ctx.response.status = 500; // This status code doesn't make it to client when there's an error.
  });

  ctx.response.type = 'application/pdf';
  ctx.response.body = fs.createReadStream('myFile.pdf');
});

And here is my client code:

async function main() {
  const request = {
    method: 'POST',
    body: JSON.stringify({ file: 'bad-file-name' }),
    headers: {
      'Content-Type': 'application/json',
      'Accept': 'application/pdf'
    }
  }

  const response = await fetch('/getFile', request);

  if (!response.ok) {
    console.log(response.status); // This is always 404 when I give a bad file name, even though I set it to 500 above. Why?
  }
}

Everything is fine when I send a proper file name, but why is the response status code always 404 even though I set it to 500 in my server code during error? Could it be that the response is already finished sending by the time my code reaches ctx.response.body = ... in which case the code in the .on('error') isn't doing anything?

Any help would be appreciated.

via saadq