Writing buffer response from resemble.js to file

I'm using node-resemble-js to compare two PNG images.

The comparison happens without issue and I get a successful/relevant response however I'm having trouble outputting the image diff.

var compareImages = function (res) {
    var diff = resemble('1.png')
        .compareTo('2.png')
        .ignoreColors()
        .onComplete(function(data){
            var png = data.getDiffImage();
            fs.writeFile('diff.png', png.data, null, function (err) {
                if (err) {
                    throw 'error writing file: ' + err;
                }
                console.log('file written');
            });
        });
};

It writes to diff.png as expected however it's not creating a valid image.

Any ideas where I'm going wrong? Feel like I'm pretty close but just unsure of final piece.

Thanks