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
	else {
		
		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;
			}
		}
	}