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 (and a co-worker) finally found what our particular problem was: do not save the font files to %TEMP%. For some reason, having the fonts saved to some other folder makes it work (for us), and saving it to anywhere inside %TEMP% makes it break.