diff --git a/src/hb-common.h b/src/hb-common.h
index eefe984dc..562b04cec 100644
--- a/src/hb-common.h
+++ b/src/hb-common.h
@@ -119,7 +119,7 @@ hb_direction_to_string (hb_direction_t direction);
 #define HB_DIRECTION_IS_FORWARD(dir)	((((unsigned int) (dir)) & ~2U) == 4)
 #define HB_DIRECTION_IS_BACKWARD(dir)	((((unsigned int) (dir)) & ~2U) == 5)
 #define HB_DIRECTION_IS_VALID(dir)	((((unsigned int) (dir)) & ~3U) == 4)
-#define HB_DIRECTION_REVERSE(dir)	((hb_direction_t) (((unsigned int) (dir)) ^ 1))
+#define HB_DIRECTION_REVERSE(dir)	((hb_direction_t) (((unsigned int) (dir)) ^ 1)) /* Direction must be valid */
 
 
 /* hb_language_t */