exist(...) never returning

[NineToeNerd]

console.log('Checking');
gfs.exist({ _id: fileId }, function (err, found) {
  console.log('Done checking');
});

The callback above is never reached (the line console.log('Done checking');). I tried fileId as a string and ObjectID.

The file is found successfully when I use:
gfs.findOne({ _id: fileId }, function (err, file) { ... });
so I'm using that for now, but I thought you might want to look into this.

NodeJS version: 9.1.0
NPM version: 5.5.0
gridfs-stream version: 1.1.1