You'll find this in script.c
        m = iMap->mapname2mapid(mapn);
        if (map[m].flag.src4instance && st->instance_id >= 0) { // Try to redirect to the instance map, not the src map
            if ((m = instance->mapid2imapid(m, st->instance_id)) < 0) {
                ShowError("buildin_monster: Trying to spawn monster (%d) on instance map (%s) without instance attached.\n", class_, mapn);
                return false;
            }
        }
 
Change to
        if ( ( m = iMap->mapname2mapid(mapn) ) == -1 ) {
            ShowWarning("buildin_monster: Attempted to spawn monster class %d on non-existing map '%s'\n",class_, mapn);
            return false;
        }
        
        if (map[m].flag.src4instance && st->instance_id >= 0) { // Try to redirect to the instance map, not the src map
            if ((m = instance->mapid2imapid(m, st->instance_id)) < 0) {
                ShowError("buildin_monster: Trying to spawn monster (%d) on instance map (%s) without instance attached.\n", class_, mapn);
                return false;
            }
        }