Resizing image with nodeJs and AWS

I am attempting to get an image from a AWS S3 bucket using nodejs, resizing it into 4 different sizes and then saving it back to the same bucket but into a folder which in turn contains 4 folders, each for the new sizes.

When I run the function, I get the following error:

Unable to resize devimageresize/diavelBlack.jpg and upload to / due to an error: Error: Stream yields empty buffer

I am relatively new to nodejs and am not sure I am writing the code correctly. What is causing this error?

This is my code:

// dependencies
var async = require('async');
var AWS = require('aws-sdk');
var gm = require('gm');
var util = require('util');


// get reference to S3 client
var s3 = new AWS.S3();

exports.handler = function(event, context) {
    // Read options from the event.
    console.log("Reading options from event:\n", util.inspect(event, {depth: 5}));
    var srcBucket = event.Records[0].s3.bucket.name;
    var srcKey    = event.Records[0].s3.object.key;
    var dstBucket = event.Records[0].s3.dst;

    var _800px = {
        width: 800,
        dstKey: 800 + srcKey,
        dstBucket: dstBucket.large
    };

    var _500px = {
        width: 500,
        dstKey: 500 + srcKey,
        dstBucket: dstBucket.medium
    };

    var _200px = {
        width: 200,
        dstKey: 200 + srcKey,
        dstBucket: dstBucket.small
    };

    var _45px = {
        width: 45,
        dstKey: 45 + srcKey,
        dstBucket: dstBucket.thumbnail
    };

    var _sizesArray = [_800px, _500px, _200px, _45px];

    var len = _sizesArray.length;

    // Sanity check: validate that source and destination are same buckets.
    if (srcBucket == dstBucket) {
        console.error("Destination bucket must match source bucket.");
    }

    // Infer the image type.
    var typeMatch = srcKey.match(/\.([^.]*)$/);
    if (!typeMatch) {
        console.error('unable to infer image type for key ' + srcKey);
        return;
    }
    var imageType = typeMatch[1];
    if (imageType != "jpg" && imageType != "png") {
        console.log('skipping non-image ' + srcKey);
        return;
    }

    // Download the image from S3, transform, and upload to same S3 bucket but different folders.
    async.waterfall([
            function download(next) {
                // Download the image from S3 into a buffer.
                s3.getObject({
                        Bucket: srcBucket,
                        Key: srcKey
                    },
                    next);
            },

            function transform(response, next) {


                for (var i = 0; i<len; i++) {

                    // Transform the image buffer in memory.
                    gm(response.Body).resize(_sizesArray[i].width)
                        .toBuffer(imageType, function(err, buffer) {
                            if (err) {
                                next(err);
                            } else {
                                next(null, response.ContentType, buffer);
                            }
                        });
                }
            },

            function upload(contentType, data, next) {

                for (var i = 0; i<len; i++) {

                    // Stream the transformed image to a different S3 bucket.
                    s3.putObject({
                            Bucket: _sizesArray[i].dstBucket,
                            Key: _sizesArray[i].dstKey,
                            Body: data,
                            ContentType: contentType
                        },
                        next);
                }
            }

        ], function (err) {
            if (err) {
                console.error(
                    'Unable to resize ' + srcBucket + '/' + srcKey +
                    ' and upload to ' + dstBucket + '/' +
                    ' due to an error: ' + err
                );
            } else {
                console.log(
                    'Successfully resized ' + srcBucket + '/' + srcKey +
                    ' and uploaded to ' + dstBucket
                );
            }

            context.done();
        }
    );
};