Customize & Windows in 21.4.6.RC1
I may not have to time to actually fix it, but I think the problem is
that Windows has a completely unspecified value of the
(face-font 'default) specifier, with the real value provided in the
fallback.
frob-face-property is probably not handling that case. It iterates
over specifier-spec-list which is nil in this case. Ah, the joys of
duplication the specifier resolving code in lisp.
Jan