Node.js: Download file from s3 and unzip it to a string

I am writing an AWS Lambda function which needs to download files from AWS S3, unzips the file and returns the content in the form of a string.

I am trying this

function getObject(key){
  var params = {
    Bucket: "my-bucket",
    Key: key
  }
  return new Promise(function (resolve, reject){
    s3.getObject(params, function (err, data){
      if(err){
        reject(err);
      }
      resolve(zlib.unzipSync(data.Body))
    })
  })
}

But getting the error

Error: incorrect header check
    at Zlib._handle.onerror (zlib.js:363:17)
    at Unzip.Zlib._processChunk (zlib.js:524:30)
    at zlibBufferSync (zlib.js:239:17)

The data looks like this

{ AcceptRanges: 'bytes',
  LastModified: 'Wed, 16 Mar 2016 04:47:10 GMT',
  ContentLength: '318',
  ETag: '"c3xxxxxxxxxxxxxxxxxxxxxxxxx"',
  ContentType: 'binary/octet-stream',
  Metadata: {},
  Body: <Buffer 50 4b 03 04 14 00 00 08 08 00 f0 ad 6f 48 95 05 00 50 84 00 00 00 b4 00 00 00 2c 00 00 00 30 30 33 32 35 2d 39 31 38 30 34 2d 37 34 33 30 39 2d 41 41 ... > 
}