diff --git a/airport_gui.c b/airport_gui.c
--- a/airport_gui.c
+++ b/airport_gui.c
@@ -2,6 +2,7 @@
 #include "openttd.h"
 #include "table/sprites.h"
 #include "table/strings.h"
+#include "functions.h"
 #include "map.h"
 #include "window.h"
 #include "gui.h"