VM836型试验js:52Uncaught(in promise)SyntaxError:JSON中位置0处的意外标记<。为什么?

async function sendURL(urls) {
  const res = await fetch("/ffmpegserver/upload/urls", 
  { method: "POST", headers: { 'Accept': 'application/json', 'Content-Type': 'application/json' }, 
  body: JSON.stringify({ urls: urls}) });
  const json = await res.json();
  return json;
}


window.onload = async () => {
  const result = await sendURL([
    "http://s5.qhres.com/static/465f1f953f1e6ff2.mp3",
    "https://file-examples-com.github.io/uploads/2017/11/file_example_MP3_700KB.mp3"
  ]);
  console.log(result);
}