diff --git a/src/g_level.cpp b/src/g_level.cpp
index 9af836035..282298e0e 100644
--- a/src/g_level.cpp
+++ b/src/g_level.cpp
@@ -1386,6 +1386,7 @@ void G_DoLoadLevel(const FString &nextmapname, int position, bool autosave, bool
 void FLevelLocals::DoLoadLevel(const FString &nextmapname, int position, bool autosave, bool newGame)
 {
 	MapName = nextmapname;
+	MapFName = MapName;
 	int i;
 
 	if (NextSkill >= 0)
diff --git a/src/g_levellocals.h b/src/g_levellocals.h
index a268fbba1..29b23c9a0 100644
--- a/src/g_levellocals.h
+++ b/src/g_levellocals.h
@@ -540,6 +540,7 @@ public:
 	int			lumpnum;
 	FString		LevelName;
 	FString		MapName;			// the lump name (E1M1, MAP01, etc)
+	FName		MapFName;			// cache name index for lump
 	FString		NextMap;			// go here when using the regular exit
 	FString		NextSecretMap;		// map to go to when used secret exit
 	FString		AuthorName;
diff --git a/src/r_data/gldefs.cpp b/src/r_data/gldefs.cpp
index b87a9b7b4..7eb88724b 100644
--- a/src/r_data/gldefs.cpp
+++ b/src/r_data/gldefs.cpp
@@ -92,11 +92,10 @@ const GlobalShaderDesc * GetGlobalShader(int shaderNum, PClass * curActor, Globa
 			}
 		}
 
-		FName MapName = level.MapName;
-		shader = mapshaders[shaderNum].CheckKey(MapName);
+		shader = mapshaders[shaderNum].CheckKey(level.MapFName);
 		if(shader && shader->shaderindex > 0)
 		{
-			addr = {int16_t(shaderNum), 1, MapName.GetIndex()};
+			addr = {int16_t(shaderNum), 1, level.MapFName.GetIndex()};
 			return shader;
 		}
 
@@ -127,7 +126,7 @@ const GlobalShaderDesc * GetGlobalShader(int shaderNum, PClass * curActor)
 
 const GlobalShaderDesc * GetGlobalShader(GlobalShaderAddr index)
 {
-	if(index.num > NUM_BUILTIN_SHADERS || index.type > 2) return &nullglobalshader;
+	if(index.num >= NUM_BUILTIN_SHADERS || index.type > 2) return &nullglobalshader;
 
 	if(index.type == 0) return &globalshaders[index.num];