diff --git a/src/highlight.c b/src/highlight.c
index 2542c7b..c373afb 100644
--- a/src/highlight.c
+++ b/src/highlight.c
@@ -3504,6 +3504,7 @@
 
 /*
  * Translate a group ID to highlight attributes.
+ * "hl_id" must be valid: > 0, caller must check.
  */
     int
 syn_id2attr(int hl_id)