"image is not a Android.Graphics.Bitmap object" Error - ZQ520 & Link-OS for Xamarin Android

I'm trying to store an image on a ZQ520 Printer, using Xamarin Android as follows:

                var image = ZebraImageFactory.GetImage(bitmap);

                printer.StoreImage("E:FOO", image, bitmap.Width, bitmap.Height);

But I'm getting an exception "image is not a Android.Graphics.Bitmap object".

I've no idea why this is the case as the image is a valid JPEG.

Any ideas are welcome