diff --git a/src/os/macosx/macos.mm b/src/os/macosx/macos.mm
index 21f58341bd..7405d06df0 100644
--- a/src/os/macosx/macos.mm
+++ b/src/os/macosx/macos.mm
@@ -15,6 +15,7 @@
 #include "../../fileio_func.h"
 #include <pthread.h>
 #include <array>
+#include <optional>
 
 #define Rect  OTTDRect
 #define Point OTTDPoint