Android : MvxImageView circular image

I have subclassed ImageView and overloaded the OnDraw() method in order to display a circular image instead of the original square one.

Now I want to change the MvxImageView to do the same. However, when I override the OnDraw method, the Bitmap property seems to be null.

   protected override void OnDraw(Android.Graphics.Canvas canvas)
    {
        if (Drawable == null)
            return;

        if (Width == 0 || Height == 0)
            return;

        Bitmap b = ((BitmapDrawable)Drawable).Bitmap;
        Bitmap bitmap = b.Copy(Bitmap.Config.Argb8888, true);

What am I doing wrong/where to get the Bitmap?

regards,

Arthur