Create NinePatch Programmatically

I'm trying to create a NinePatch image programmatically by using the code found at: https://gist.github.com/briangriffey/4391807

When doing:

NinePatch patch = new NinePatch(bitmap, buffer.ToArray<byte>(), srcName); 

I'm getting an error saying:

System.InvalidCastException: Unable to cast from 'java/nio/ByteArrayBuffer' to '[B'.

Can anyone help me with this error?