diff --git a/Source/Platform.h b/Source/Platform.h
index ae0781c6..b88b77ce 100644
--- a/Source/Platform.h
+++ b/Source/Platform.h
@@ -395,6 +395,7 @@ typedef WORD LANGID;
 #  define MB_SETFOREGROUND 0x10000
 #  define MB_TOPMOST 0x40000
 #  define MB_RIGHT 0x80000
+#  define MB_RTLREADING 0x100000
 #endif
 
 #ifndef IDOK