createObjectURL error: `argument must be an instance of Blob. Received an instance of Blob`

I have an express route which contains the following:

let result = await fetch("http://someurl");
result = await result.blob();
console.log(result)
const img = URL.createObjectURL(result);

This produces the following error (on line 4):

TypeError: The "obj" argument must be an instance of Blob. Received an instance of Blob

What's going on here?

If it helps, the result of the console.log is:

Blob {
  [Symbol(type)]: 'image/png',
  [Symbol(buffer)]: <Buffer 89 50 4e  ... 481501 more bytes>
}

and the fetch route goes to another express app which calls res.sendFile(myfilepath)