diff --git a/src/message.c b/src/message.c
index 6daa365..0a2fe99 100644
--- a/src/message.c
+++ b/src/message.c
@@ -3749,6 +3749,7 @@
 
 /*
  * Get string argument from "idxp" entry in "tvs".  First entry is 1.
+ * Returns NULL for an error.
  */
     static char *
 tv_str(tvs, idxp)