Can't save arrowheads.ai file to create custom arrowhead
I'm creating a custom arrowhead, following the steps outlined here.
I open arrowheads.ai, create the arrowhead, add it to the symbols palette, name it, etc.
But when I go to save the file arrowheads.ai, I can't save the file, getting this error message:

I'm working on AI CS5, on Windows 7. Thanks for any help on this.