diff --git a/headers/os/app/Message.h b/headers/os/app/Message.h
index 038d5166f3..d406a45814 100644
--- a/headers/os/app/Message.h
+++ b/headers/os/app/Message.h
@@ -14,6 +14,7 @@
 #include <Flattenable.h>
 #include <OS.h>
 #include <Rect.h>
+#include <Size.h>
 
 #include <AppDefs.h>		/* For convenience */
 #include <TypeConstants.h>	/* For convenience */