无法从S3存储桶下载文件

Question

downloadFileFromS3: (res, fileName) => {
    const params = { Bucket: process.env.AMZ_BUCKET, Key: fileName } // keyname can be a filename
    s3.getObject(params, function (err, data) {
      if (err) {
        return res.send({ error: err })
      }
      return res.download(data)
    })
  },

err：Error [TypeError]：文件名必须为字符串