WPF - Saving font to disk, then instantiating GlyphTypeface for font throws exception. Why?

Question

I am mucking about with WPF glyphs and documents and have run into a null reference exception in the .Net 4 framework.

I extract and save true-type fonts to disk as

Answer 1

I ended up using the workaround of saving each font to its own folder (using the font name for the folder name). The exception went away, so I guess we can chalk this up to a bug in .Net.