diff --git a/src/lua_baselib.c b/src/lua_baselib.c
index 2a82ec512..d4d6e1a6d 100644
--- a/src/lua_baselib.c
+++ b/src/lua_baselib.c
@@ -2772,11 +2772,28 @@ static int lib_gAddGametype(lua_State *L)
 	return 0;
 }
 
+static int Lcheckmapnumber (lua_State *L, int idx)
+{
+	if (ISINLEVEL)
+		return luaL_optinteger(L, idx, gamemap);
+	else
+	{
+		if (lua_isnoneornil(L, idx))
+		{
+			return luaL_error(L,
+					"G_BuildMapName can only be used "
+					"without a parameter while in a level."
+			);
+		}
+		else
+			return luaL_checkinteger(L, idx);
+	}
+}
+
 static int lib_gBuildMapName(lua_State *L)
 {
-	INT32 map = luaL_optinteger(L, 1, gamemap);
+	INT32 map = Lcheckmapnumber(L, 1);
 	//HUDSAFE
-	INLEVEL
 	lua_pushstring(L, G_BuildMapName(map));
 	return 1;
 }
diff --git a/src/lua_script.h b/src/lua_script.h
index 8f27dcb4c..7d8aaa282 100644
--- a/src/lua_script.h
+++ b/src/lua_script.h
@@ -100,7 +100,10 @@ void COM_Lua_f(void);
 // uncomment if you want seg_t/node_t in Lua
 // #define HAVE_LUA_SEGS
 
-#define INLEVEL if (gamestate != GS_LEVEL && !titlemapinaction)\
+#define ISINLEVEL \
+	(gamestate == GS_LEVEL || titlemapinaction)
+
+#define INLEVEL if (! ISINLEVEL)\
 return luaL_error(L, "This can only be used in a level!");
 
 #endif