The code at https://github.com/libtcod/libtcod/blob/main/src/libtcod/tileset_fallback.c has been tested with Ubuntu but segfaults with Debain x86_64, possibly due to bad error handing. I can't verify that it's actually this code that fails, just that not giving a tileset causes the failure on Debain. I don't have the resources to debug this at the moment.