Tileset interaction with unknown objects

[Atopian]

So, I use a tileset. I know this isn't popular with many people, but I got too used to the old ADoM ASCII to be able to use Angband-type ASCII.

Anyway, I like the Adam Bolt tileset, but that hasn't been updated for a long time, which causes some problems with unknown objects / monsters.

Sometimes when an object or monster is unknown (e.g.: spellbooks for realms added after ZAngband, potions most 'p' or 'h' enemies), there is no problem - the ASCII character shows through instead.
But sometimes, it just shows as a black square (e.g.: bolts, certain monsters I can never remember examples of), which can have ...unfortunate consequences.

Does anyone know how I could adjust display behaviour so that, when using a tileset, things unknown to that tileset would consistently show up as their ASCII instead of as blank black squares?