diff --git a/include/WinStd/Common.h b/include/WinStd/Common.h
index c1831e2a..26ff5c20 100644
--- a/include/WinStd/Common.h
+++ b/include/WinStd/Common.h
@@ -188,7 +188,7 @@ private:
 
 
 #ifndef _FormatMessage_format_string_
-#define _FormatMessage_format_string_
+#define _FormatMessage_format_string_ _In_z_
 #endif