diff --git a/src/d_dehacked.cpp b/src/d_dehacked.cpp
index 1065836c1..87f1d589e 100644
--- a/src/d_dehacked.cpp
+++ b/src/d_dehacked.cpp
@@ -2431,12 +2431,12 @@ int D_LoadDehLumps(DehLumpSource source)
 	{
 		const int filenum = Wads.GetLumpFile(lumpnum);
 		
-		if (FromIWAD == source && filenum > FWadCollection::IWAD_FILENUM)
+		if (FromIWAD == source && filenum > Wads.GetIwadNum())
 		{
 			// No more DEHACKED lumps in IWAD
 			break;
 		}
-		else if (FromPWADs == source && filenum <= FWadCollection::IWAD_FILENUM)
+		else if (FromPWADs == source && filenum <= Wads.GetIwadNum())
 		{
 			// Skip DEHACKED lumps from IWAD
 			continue;
diff --git a/src/d_iwad.cpp b/src/d_iwad.cpp
index 202eb2b5e..512e1a905 100644
--- a/src/d_iwad.cpp
+++ b/src/d_iwad.cpp
@@ -688,8 +688,10 @@ int FIWadManager::IdentifyVersion (TArray<FString> &wadfiles, const char *iwad,
 	D_AddFile (wadfiles, zdoom_wad);
 
 	// [SP] Load non-free assets if available. This must be done before the IWAD.
-	if (optional_wad)
-		D_AddFile(wadfiles, optional_wad);
+	if (D_AddFile(wadfiles, optional_wad))
+		Wads.SetIwadNum(2);
+	else
+		Wads.SetIwadNum(1);
 
 	if (picks[pick].mRequiredPath.IsNotEmpty())
 	{
diff --git a/src/d_main.cpp b/src/d_main.cpp
index 6edb88fdc..923d510ec 100644
--- a/src/d_main.cpp
+++ b/src/d_main.cpp
@@ -1992,9 +1992,9 @@ static FString CheckGameInfo(TArray<FString> & pwads)
 
 static void SetMapxxFlag()
 {
-	int lump_name = Wads.CheckNumForName("MAP01", ns_global, FWadCollection::IWAD_FILENUM);
-	int lump_wad = Wads.CheckNumForFullName("maps/map01.wad", FWadCollection::IWAD_FILENUM);
-	int lump_map = Wads.CheckNumForFullName("maps/map01.map", FWadCollection::IWAD_FILENUM);
+	int lump_name = Wads.CheckNumForName("MAP01", ns_global, Wads.GetIwadNum());
+	int lump_wad = Wads.CheckNumForFullName("maps/map01.wad", Wads.GetIwadNum());
+	int lump_map = Wads.CheckNumForFullName("maps/map01.map", Wads.GetIwadNum());
 
 	if (lump_name >= 0 || lump_wad >= 0 || lump_map >= 0) gameinfo.flags |= GI_MAPxx;
 }
diff --git a/src/g_game.cpp b/src/g_game.cpp
index d48b40b6a..d3dd47f46 100644
--- a/src/g_game.cpp
+++ b/src/g_game.cpp
@@ -2195,7 +2195,7 @@ static void PutSaveWads (FSerializer &arc)
 	const char *name;
 
 	// Name of IWAD
-	name = Wads.GetWadName (FWadCollection::IWAD_FILENUM);
+	name = Wads.GetWadName (Wads.GetIwadNum());
 	arc.AddString("Game WAD", name);
 
 	// Name of wad the map resides in
diff --git a/src/gl/data/gl_data.cpp b/src/gl/data/gl_data.cpp
index a055523fc..d0b1a4a28 100644
--- a/src/gl/data/gl_data.cpp
+++ b/src/gl/data/gl_data.cpp
@@ -88,13 +88,13 @@ void AdjustSpriteOffsets()
 	for (int i = 0; i < numtex; i++)
 	{
 		if (Wads.GetLumpFile(i) > 1) break; // we are past the IWAD
-		if (Wads.GetLumpNamespace(i) == ns_sprites && Wads.GetLumpFile(i) == FWadCollection::IWAD_FILENUM)
+		if (Wads.GetLumpNamespace(i) == ns_sprites && Wads.GetLumpFile(i) == Wads.GetIwadNum())
 		{
 			char str[9];
 			Wads.GetLumpName(str, i);
 			str[8] = 0;
 			FTextureID texid = TexMan.CheckForTexture(str, FTexture::TEX_Sprite, 0);
-			if (texid.isValid() && Wads.GetLumpFile(TexMan[texid]->SourceLump) > FWadCollection::IWAD_FILENUM)
+			if (texid.isValid() && Wads.GetLumpFile(TexMan[texid]->SourceLump) > Wads.GetIwadNum())
 			{
 				// This texture has been replaced by some PWAD.
 				memcpy(&sprid, str, 4);
@@ -137,9 +137,9 @@ void AdjustSpriteOffsets()
 				if (lumpnum >= 0 && lumpnum < Wads.GetNumLumps())
 				{
 					int wadno = Wads.GetLumpFile(lumpnum);
-					if ((iwadonly && wadno==FWadCollection::IWAD_FILENUM) || (!iwadonly && wadno == ofslumpno))
+					if ((iwadonly && wadno==Wads.GetIwadNum()) || (!iwadonly && wadno == ofslumpno))
 					{
-						if (wadno == FWadCollection::IWAD_FILENUM && !forced && iwadonly)
+						if (wadno == Wads.GetIwadNum() && !forced && iwadonly)
 						{
 							memcpy(&sprid, &tex->Name[0], 4);
 							if (donotprocess.CheckKey(sprid)) continue;	// do not alter sprites that only get partially replaced.
diff --git a/src/gl/textures/gl_texture.cpp b/src/gl/textures/gl_texture.cpp
index af1a6178f..e5a91fe14 100644
--- a/src/gl/textures/gl_texture.cpp
+++ b/src/gl/textures/gl_texture.cpp
@@ -614,7 +614,7 @@ void gl_ParseBrightmap(FScanner &sc, int deflump)
 
 		if (lumpnum != -1)
 		{
-			if (iwad && Wads.GetLumpFile(lumpnum) <= FWadCollection::IWAD_FILENUM) useme = true;
+			if (iwad && Wads.GetLumpFile(lumpnum) <= Wads.GetIwadNum()) useme = true;
 			if (thiswad && Wads.GetLumpFile(lumpnum) == deflump) useme = true;
 		}
 		if (!useme) return;
diff --git a/src/menu/loadsavemenu.cpp b/src/menu/loadsavemenu.cpp
index f76654165..bec3082b1 100644
--- a/src/menu/loadsavemenu.cpp
+++ b/src/menu/loadsavemenu.cpp
@@ -213,7 +213,7 @@ void FSavegameManager::ReadSaveStrings()
 							// old, incompatible savegame. List as not usable.
 							oldVer = true;
 						}
-						else if (iwad.CompareNoCase(Wads.GetWadName(FWadCollection::IWAD_FILENUM)) == 0)
+						else if (iwad.CompareNoCase(Wads.GetWadName(Wads.GetIwadNum())) == 0)
 						{
 							missing = !G_CheckSaveGameWads(arc, false);
 						}
diff --git a/src/menu/menudef.cpp b/src/menu/menudef.cpp
index d97705b34..de9c8bc92 100644
--- a/src/menu/menudef.cpp
+++ b/src/menu/menudef.cpp
@@ -1014,7 +1014,7 @@ void M_ParseMenuDefs()
 	atterm(	DeinitMenus);
 	DeinitMenus();
 
-	int IWADMenu = Wads.CheckNumForName("MENUDEF", ns_global, FWadCollection::IWAD_FILENUM);
+	int IWADMenu = Wads.CheckNumForName("MENUDEF", ns_global, Wads.GetIwadNum());
 
 	while ((lump = Wads.FindLump ("MENUDEF", &lastlump)) != -1)
 	{
diff --git a/src/p_user.cpp b/src/p_user.cpp
index 104e7d241..520861651 100644
--- a/src/p_user.cpp
+++ b/src/p_user.cpp
@@ -962,7 +962,7 @@ void APlayerPawn::BeginPlay ()
 		int wadnorm = Wads.GetLumpFile(spritenorm);
 		int wadcrouch = Wads.GetLumpFile(spritenorm);
 		
-		if (wadnorm > FWadCollection::IWAD_FILENUM && wadcrouch <= FWadCollection::IWAD_FILENUM) 
+		if (wadnorm > Wads.GetIwadNum() && wadcrouch <= Wads.GetIwadNum())
 		{
 			// Question: Add an option / disable crouching or do what?
 			crouchsprite = 0;
diff --git a/src/v_font.cpp b/src/v_font.cpp
index 30a3f45f6..68422cd74 100644
--- a/src/v_font.cpp
+++ b/src/v_font.cpp
@@ -2275,7 +2275,7 @@ void V_InitCustomFonts()
 					{
 						*p = TexMan[texid];
 					}
-					else if (Wads.GetLumpFile(sc.LumpNum) >= Wads.IWAD_FILENUM)
+					else if (Wads.GetLumpFile(sc.LumpNum) >= Wads.GetIwadNum())
 					{
 						// Print a message only if this isn't in zdoom.pk3
 						sc.ScriptMessage("%s: Unable to find texture in font definition for %s", sc.String, namebuffer.GetChars());
diff --git a/src/w_wad.cpp b/src/w_wad.cpp
index f1e633970..e4a8d1af0 100644
--- a/src/w_wad.cpp
+++ b/src/w_wad.cpp
@@ -281,7 +281,7 @@ void FWadCollection::AddFile (const char *filename, FileReader *wadinfo)
 			lump_p->wadnum = Files.Size();
 		}
 
-		if (Files.Size() == IWAD_FILENUM && gameinfo.gametype == GAME_Strife && gameinfo.flags & GI_SHAREWARE)
+		if (Files.Size() == GetIwadNum() && gameinfo.gametype == GAME_Strife && gameinfo.flags & GI_SHAREWARE)
 		{
 			resfile->FindStrifeTeaserVoices();
 		}
@@ -859,7 +859,7 @@ void FWadCollection::RenameSprites ()
 		if (LumpInfo[i].lump->Namespace == ns_sprites)
 		{
 			// Only sprites in the IWAD normally get renamed
-			if (renameAll || LumpInfo[i].wadnum == IWAD_FILENUM)
+			if (renameAll || LumpInfo[i].wadnum == GetIwadNum())
 			{
 				for (int j = 0; j < numrenames; ++j)
 				{
@@ -921,7 +921,7 @@ void FWadCollection::RenameNerve ()
 	static const uint8_t nerve[16] = { 0x96, 0x7d, 0x5a, 0xe2, 0x3d, 0xaf, 0x45, 0x19,
 		0x62, 0x12, 0xae, 0x1b, 0x60, 0x5d, 0xa3, 0xb0 };
 	size_t nervesize = 3819855; // NERVE.WAD's file size
-	int w = IWAD_FILENUM;
+	int w = GetIwadNum();
 	while (++w < GetNumWads())
 	{
 		FileReader *fr = GetFileReader(w);
@@ -985,7 +985,7 @@ void FWadCollection::FixMacHexen()
 		return;
 	}
 
-	FileReader* const reader = GetFileReader(IWAD_FILENUM);
+	FileReader* const reader = GetFileReader(GetIwadNum());
 	const long iwadSize = reader->GetLength();
 
 	static const long DEMO_SIZE = 13596228;
@@ -1038,8 +1038,8 @@ void FWadCollection::FixMacHexen()
 	// Hexen Beta is very similar to Demo but it has MAP41: Maze at the end of the WAD
 	// So keep this map if it's present but discard all extra lumps
 
-	const int lastLump = GetLastLump(IWAD_FILENUM) - (isBeta ? 12 : 0);
-	assert(GetFirstLump(IWAD_FILENUM) + 299 < lastLump);
+	const int lastLump = GetLastLump(GetIwadNum()) - (isBeta ? 12 : 0);
+	assert(GetFirstLump(GetIwadNum()) + 299 < lastLump);
 
 	for (int i = lastLump - EXTRA_LUMPS + 1; i <= lastLump; ++i)
 	{
diff --git a/src/w_wad.h b/src/w_wad.h
index 52be4e79f..6fdc62f84 100644
--- a/src/w_wad.h
+++ b/src/w_wad.h
@@ -134,7 +134,8 @@ public:
 	~FWadCollection ();
 
 	// The wadnum for the IWAD
-	enum { IWAD_FILENUM = 1 };
+	int GetIwadNum() { return IwadIndex; }
+	void SetIwadNum(int x) { IwadIndex = x; }
 
 	void InitMultipleFiles (TArray<FString> &filenames);
 	void AddFile (const char *filename, FileReader *wadinfo = NULL);
@@ -224,6 +225,8 @@ protected:
 	uint32_t NumLumps;					// Not necessarily the same as LumpInfo.Size()
 	uint32_t NumWads;
 
+	int IwadIndex;
+
 	void SkinHack (int baselump);
 	void InitHashChains ();								// [RH] Set up the lumpinfo hashing
 
diff --git a/src/win32/st_start.cpp b/src/win32/st_start.cpp
index 0fb89d581..5ba3848c7 100644
--- a/src/win32/st_start.cpp
+++ b/src/win32/st_start.cpp
@@ -1108,7 +1108,7 @@ void ST_Endoom()
 		exit(0);
 	}
 
-	if (Wads.GetLumpFile(endoom_lump) == FWadCollection::IWAD_FILENUM && showendoom == 2)
+	if (Wads.GetLumpFile(endoom_lump) == Wads.GetIwadNum() && showendoom == 2)
 	{
 		// showendoom==2 means to show only lumps from PWADs.
 		exit(0);