diff --git a/src/p_acs.cpp b/src/p_acs.cpp
index fd263490a..2ff70c9e3 100644
--- a/src/p_acs.cpp
+++ b/src/p_acs.cpp
@@ -1581,20 +1581,28 @@ void FBehavior::StaticSerializeModuleStates (FArchive &arc)
 	for (modnum = 0; modnum < StaticModules.Size(); ++modnum)
 	{
 		FBehavior *module = StaticModules[modnum];
+		int ModSize = module->GetDataSize();
 
 		if (arc.IsStoring())
 		{
 			arc.WriteString (module->ModuleName);
+			if (SaveVersion >= 4516) arc << ModSize;
 		}
 		else
 		{
 			char *modname = NULL;
 			arc << modname;
+			if (SaveVersion >= 4516) arc << ModSize;
 			if (stricmp (modname, module->ModuleName) != 0)
 			{
 				delete[] modname;
 				I_Error ("Level was saved with a different set of ACS modules.");
 			}
+			else if (ModSize != module->GetDataSize())
+			{
+				delete[] modname;
+				I_Error("ACS module %s has changed from what was saved. (Have %d bytes, save has %d bytes)", module->ModuleName, module->GetDataSize(), ModSize);
+			}
 			delete[] modname;
 		}
 		module->SerializeVars (arc);
diff --git a/src/p_acs.h b/src/p_acs.h
index 02544e367..88016f0db 100644
--- a/src/p_acs.h
+++ b/src/p_acs.h
@@ -308,6 +308,7 @@ public:
 	int GetScriptIndex (const ScriptPtr *ptr) const { ptrdiff_t index = ptr - Scripts; return index >= NumScripts ? -1 : (int)index; }
 	ScriptPtr *GetScriptPtr(int index) const { return index >= 0 && index < NumScripts ? &Scripts[index] : NULL; }
 	int GetLumpNum() const { return LumpNum; }
+	int GetDataSize() const { return DataSize; }
 	const char *GetModuleName() const { return ModuleName; }
 	ACSProfileInfo *GetFunctionProfileData(int index) { return index >= 0 && index < NumFunctions ? &FunctionProfileData[index] : NULL; }
 	ACSProfileInfo *GetFunctionProfileData(ScriptFunction *func) { return GetFunctionProfileData((int)(func - (ScriptFunction *)Functions)); }