diff --git a/src/compatibility.cpp b/src/compatibility.cpp
index 947b7ffaef..8c3cc16c5c 100644
--- a/src/compatibility.cpp
+++ b/src/compatibility.cpp
@@ -465,7 +465,7 @@ void CheckCompatibility(MapData *map)
 	// When playing Doom IWAD levels force COMPAT_SHORTTEX and COMPATF_LIGHT.
 	// I'm not sure if the IWAD maps actually need COMPATF_LIGHT but it certainly does not hurt.
 	// TNT's MAP31 also needs COMPATF_STAIRINDEX but that only gets activated for TNT.WAD.
-	if (Wads.GetLumpFile(map->lumpnum) == 1 && (gameinfo.flags & GI_COMPATSHORTTEX) && level.maptype == MAPTYPE_DOOM)
+	if (Wads.GetLumpFile(map->lumpnum) == Wads.GetIwadNum() && (gameinfo.flags & GI_COMPATSHORTTEX) && level.maptype == MAPTYPE_DOOM)
 	{
 		ii_compatflags = COMPATF_SHORTTEX|COMPATF_LIGHT;
 		if (gameinfo.flags & GI_COMPATSTAIRS) ii_compatflags |= COMPATF_STAIRINDEX;