diff --git a/src/deh_soc.c b/src/deh_soc.c
index 8e4a8082f..d30957647 100644
--- a/src/deh_soc.c
+++ b/src/deh_soc.c
@@ -1132,19 +1132,14 @@ void readlevelheader(MYFILE *f, char * name)
 	// compatmode map number system:
 	// 0-1034 correspond to MAP01-MAPZZ,
 	// longname maps are appended starting at NUMMAPS
-	if (exnum)
+	if (!exnum)
 	{
-		exnum--;
-		kartmap2native[exnum] = num;
-		nativemap2kart[num] = exnum;
-	}
-	else
-	{
-		kartmap2native[nextexnum] = num;
-		nativemap2kart[num] = nextexnum;
-		if (++nextexnum >= NEXTMAP_SPECIAL)
+		exnum = ++nextexnum;
+		if (exnum >= NEXTMAP_SPECIAL)
 			I_Error("Ran out of compatibility map slots");
 	}
+	kartmap2native[exnum-1] = num;
+	nativemap2kart[num] = exnum-1;
 
 	if (mapheaderinfo[num]->lumpname == NULL)
 	{
diff --git a/src/g_game.c b/src/g_game.c
index 7b67bcf6e..3be9cce6f 100644
--- a/src/g_game.c
+++ b/src/g_game.c
@@ -340,7 +340,6 @@ boolean precache = true; // if true, load all graphics at start
 
 INT16 prevmap, nextmap;
 
-// initialized to -1 in G_LoadGameSettings
 INT16 kartmap2native[NEXTMAP_SPECIAL] = {0}, nativemap2kart[NEXTMAP_SPECIAL] = {0};
 INT16 nextexnum = NUMMAPS;
 
@@ -796,17 +795,17 @@ INT32 G_MapNumber(const char * name)
 // convert kart map number to native map number
 INT16 G_KartMapToNative(INT16 mapnum)
 {
-	if (mapnum >= 0 && mapnum < NEXTMAP_SPECIAL)
-		return kartmap2native[mapnum];
-	return -1;
+	if (mapnum > 0 && mapnum < NEXTMAP_SPECIAL)
+		return kartmap2native[mapnum-1]+1;
+	return 0;
 }
 
 // convert native map number to kart
 INT16 G_NativeMapToKart(INT16 mapnum)
 {
-	if (mapnum >= 0 && mapnum < NEXTMAP_SPECIAL)
-		return nativemap2kart[mapnum];
-	return -1;
+	if (mapnum > 0 && mapnum < NEXTMAP_SPECIAL)
+		return nativemap2kart[mapnum-1]+1;
+	return 0;
 }
 
 /** Clips the console player's mouse aiming to the current view.
@@ -4359,9 +4358,6 @@ void G_LoadGameSettings(void)
 {
 	// initialize free sfx slots for skin sounds
 	S_InitRuntimeSounds();
-
-	for (size_t i = 0; i < NEXTMAP_SPECIAL; i++)
-		kartmap2native[i] = nativemap2kart[i] = -1;
 }
 
 #define GD_VERSIONCHECK 0xBA5ED444
diff --git a/src/lua_baselib.c b/src/lua_baselib.c
index 8467652da..cdeb7f183 100644
--- a/src/lua_baselib.c
+++ b/src/lua_baselib.c
@@ -3164,7 +3164,7 @@ static int lib_gAddGametype(lua_State *L)
 static int Lcheckmapnumber (lua_State *L, int idx, const char *fun)
 {
 	if (ISINLEVEL)
-		return luaL_optinteger(L, idx, lua_compatmode ? G_NativeMapToKart(gamemap-1)+1 : gamemap);
+		return luaL_optinteger(L, idx, lua_compatmode ? G_NativeMapToKart(gamemap) : gamemap);
 	else
 	{
 		if (lua_isnoneornil(L, idx))
@@ -3183,7 +3183,7 @@ static int lib_gBuildMapName(lua_State *L)
 {
 	INT32 map = Lcheckmapnumber(L, 1, "G_BuildMapName");
 	//HUDSAFE
-	lua_pushstring(L, G_BuildMapName(lua_compatmode ? G_KartMapToNative(map-1)+1 : map));
+	lua_pushstring(L, G_BuildMapName(lua_compatmode ? G_KartMapToNative(map) : map));
 	return 1;
 }
 
@@ -3350,7 +3350,7 @@ static int lib_gSetCustomExitVars(lua_State *L)
 		nextmapoverride = (INT16)luaL_optinteger(L, 1, 0);
 		skipstats = (INT16)luaL_optinteger(L, 2, 0);
 		if (lua_compatmode && nextmapoverride)
-			nextmapoverride = G_KartMapToNative(nextmapoverride-1)+1;
+			nextmapoverride = G_KartMapToNative(nextmapoverride);
 	}
 
 	return 0;
@@ -3373,7 +3373,7 @@ static int lib_gIsSpecialStage(lua_State *L)
 	INT32 mapnum = Lcheckmapnumber(L, 1, "G_IsSpecialStage");
 	//HUDSAFE
 	INLEVEL
-	lua_pushboolean(L, G_IsSpecialStage(lua_compatmode ? G_KartMapToNative(mapnum-1)+1 : mapnum));
+	lua_pushboolean(L, G_IsSpecialStage(lua_compatmode ? G_KartMapToNative(mapnum) : mapnum));
 	return 1;
 }
 
diff --git a/src/lua_hooklib.c b/src/lua_hooklib.c
index a900f2331..aa4a08f83 100644
--- a/src/lua_hooklib.c
+++ b/src/lua_hooklib.c
@@ -647,7 +647,7 @@ void LUA_HookGamemap(int hook_type)
 	if (prepare_hook(&hook, 0, hook_type))
 	{
 		lua_pushinteger(gL, gamemap);
-		lua_pushinteger(gL, G_NativeMapToKart(gamemap-1)+1);
+		lua_pushinteger(gL, G_NativeMapToKart(gamemap));
 		init_hook_call(&hook, 0, res_none);
 		hook.values = 1;
 		call_mapped_gamemap(&hook, &hookIds[hook.hook_type]);
diff --git a/src/lua_maplib.c b/src/lua_maplib.c
index 86f182222..e945c74ab 100644
--- a/src/lua_maplib.c
+++ b/src/lua_maplib.c
@@ -2521,10 +2521,10 @@ static int lib_getMapheaderinfo(lua_State *L)
 	lua_remove(L, 1); // dummy userdata table is unused.
 	if (lua_isnumber(L, 1))
 	{
-		INT32 i = lua_tointeger(L, 1)-1;
+		INT32 i = lua_tointeger(L, 1);
 		if (lua_compatmode)
 			i = G_KartMapToNative(i);
-		if (i < 0 || i >= nummapheaders)
+		if (--i < 0 || i >= nummapheaders)
 			return 0;
 		LUA_PushUserdata(L, mapheaderinfo[i], META_MAPHEADER);
 		//CONS_Printf(mapheaderinfo[i]->lvlttl);
diff --git a/src/lua_script.c b/src/lua_script.c
index 53ec8c185..f203d765c 100644
--- a/src/lua_script.c
+++ b/src/lua_script.c
@@ -157,7 +157,7 @@ boolean lua_compatmode = false;
 int LUA_PushGlobals(lua_State *L, const char *word)
 {
 	if (fastcmp(word,"gamemap")) {
-		lua_pushinteger(L, lua_compatmode ? G_NativeMapToKart(gamemap-1)+1 : gamemap);
+		lua_pushinteger(L, lua_compatmode ? G_NativeMapToKart(gamemap) : gamemap);
 		return 1;
 	} else if (fastcmp(word,"udmf")) {
 		lua_pushboolean(L, udmf);