dynamically assigning an image to a Button crashes Android only

I'm doing this in a callback from a SignanturePad

buttonCustomerSignature.Image = new FileImageSource { File = sig.Filename };

this works fine on iOS. But on Android it hangs and eventually crashes with

No Package identifier when getting value for resource number 0x0000000d

on Android, sig.Filename is /data/data/m.transport.Android.Alpha/cache/tmp5637bbe6.png and is a valid png that exists.

Should this work? Do I need to do something different on Android in order for FileImageSource to resolve that path, or should I use a different location for storing the image file?