diff --git a/src/hb-open-type-private.hh b/src/hb-open-type-private.hh
index 4fea4d451..3273b8611 100644
--- a/src/hb-open-type-private.hh
+++ b/src/hb-open-type-private.hh
@@ -32,6 +32,7 @@
 #include "hb-blob.h"
 
 
+/* Table/script/language-system/feature/... not found */
 #define NO_INDEX		((unsigned int) 0xFFFF)