The mingw-w64 toolchain bizarrely substitutes scm_to_locale_string()
for scm_to_utf8_string(). This results in latin1 (yeah, "locale" is
a lie) instead of utf8 which causes an assertion in
g_utf8_collate_key().
Perhaps equally bizarre, the compiler doesn't make the substitution
with scm_to_utf8_stringn(), so use that instead.