diff --git a/src/gl/shaders/gl_shader.cpp b/src/gl/shaders/gl_shader.cpp
index b7d8335f8..327ac5fd4 100644
--- a/src/gl/shaders/gl_shader.cpp
+++ b/src/gl/shaders/gl_shader.cpp
@@ -60,15 +60,15 @@ bool FShader::Load(const char * name, const char * vert_prog_lump, const char *
 	static char buffer[10000];
 	FString error;
 
-	int i_lump = Wads.CheckNumForFullName("shaders/glsl/shaderdefs.i");
+	int i_lump = Wads.CheckNumForFullName("shaders/glsl/shaderdefs.i", 0);
 	if (i_lump == -1) I_Error("Unable to load 'shaders/glsl/shaderdefs.i'");
 	FMemLump i_data = Wads.ReadLump(i_lump);
 
-	int vp_lump = Wads.CheckNumForFullName(vert_prog_lump);
+	int vp_lump = Wads.CheckNumForFullName(vert_prog_lump, 0);
 	if (vp_lump == -1) I_Error("Unable to load '%s'", vert_prog_lump);
 	FMemLump vp_data = Wads.ReadLump(vp_lump);
 
-	int fp_lump = Wads.CheckNumForFullName(frag_prog_lump);
+	int fp_lump = Wads.CheckNumForFullName(frag_prog_lump, 0);
 	if (fp_lump == -1) I_Error("Unable to load '%s'", frag_prog_lump);
 	FMemLump fp_data = Wads.ReadLump(fp_lump);
 
diff --git a/src/gl/shaders/gl_shaderprogram.cpp b/src/gl/shaders/gl_shaderprogram.cpp
index 1ed19913a..7a4690874 100644
--- a/src/gl/shaders/gl_shaderprogram.cpp
+++ b/src/gl/shaders/gl_shaderprogram.cpp
@@ -89,7 +89,7 @@ void FShaderProgram::CreateShader(ShaderType type)
 
 void FShaderProgram::Compile(ShaderType type, const char *lumpName, const char *defines, int maxGlslVersion)
 {
-	int lump = Wads.CheckNumForFullName(lumpName);
+	int lump = Wads.CheckNumForFullName(lumpName, 0);
 	if (lump == -1) I_FatalError("Unable to load '%s'", lumpName);
 	FString code = Wads.ReadLump(lump).GetString().GetChars();
 	Compile(type, lumpName, code, defines, maxGlslVersion);
diff --git a/src/scripting/zscript/zcc_parser.cpp b/src/scripting/zscript/zcc_parser.cpp
index 78458876a..872ab2cc8 100644
--- a/src/scripting/zscript/zcc_parser.cpp
+++ b/src/scripting/zscript/zcc_parser.cpp
@@ -316,6 +316,8 @@ static void DoParse(int lumpnum)
 	FScanner sc;
 	void *parser;
 	ZCCToken value;
+	auto baselump = lumpnum;
+	auto fileno = Wads.GetLumpFile(lumpnum);
 
 	parser = ZCCParseAlloc(malloc);
 	ZCCParseState state;
@@ -344,6 +346,13 @@ static void DoParse(int lumpnum)
 		}
 		else
 		{
+			auto fileno2 = Wads.GetLumpFile(lumpnum);
+			if (fileno == 0 && fileno2 != 0)
+			{
+				I_FatalError("File %s is overriding core lump %s.",
+					Wads.GetWadFullName(Wads.GetLumpFile(baselump)), Includes[i].GetChars());
+			}
+
 			ParseSingleFile(nullptr, lumpnum, parser, state);
 		}
 	}