diff --git a/ai_old.c b/ai_old.c
--- a/ai_old.c
+++ b/ai_old.c
@@ -1,5 +1,6 @@
 #include "stdafx.h"
 #include "openttd.h"
+#include "functions.h"
 #include "map.h"
 #include "tile.h"
 #include "player.h"