diff --git a/aegisub/src/libauto3/auto3.c b/aegisub/src/libauto3/auto3.c
index 00224077c..fcbfd3eb2 100644
--- a/aegisub/src/libauto3/auto3.c
+++ b/aegisub/src/libauto3/auto3.c
@@ -344,6 +344,7 @@ static int Auto3ParseConfigData(lua_State *L, struct Auto3Interpreter *script, c
 
 
 // Keeping this file a bit shorter: put all functions called from Lua into a separate file
+#define BUILDING_AUTO3_C
 #include "callables.c"
 
 
diff --git a/aegisub/src/libauto3/callables.c b/aegisub/src/libauto3/callables.c
index c2cfa8154..41482f8d8 100644
--- a/aegisub/src/libauto3/callables.c
+++ b/aegisub/src/libauto3/callables.c
@@ -34,6 +34,9 @@
 //
 
 // DO NOT compile this file separately! It's included as part of auto3.c
+#ifndef BUILDING_AUTO3_C
+# error callables.c can not be compiled separately. It is included as part of auto3.c.
+#endif
 
 
 static struct Auto3Interpreter* GetScriptObject(lua_State *L)