diff --git a/src/libknot/packet/pkt.h b/src/libknot/packet/pkt.h
index 7e7a3fe2e66d940dd753438d8779e9c4c8255f32..f37b0c6149a48a3b4d48c6a62390819897eda2ed 100644
--- a/src/libknot/packet/pkt.h
+++ b/src/libknot/packet/pkt.h
@@ -303,7 +303,7 @@ int knot_pkt_parse_payload(knot_pkt_t *pkt, unsigned flags);
  *
  * \param pkt Packet to get the response code from.
  *
- * \return
+ * \return Whole extended RCODE (0 if pkt == NULL).
  */
 uint16_t knot_pkt_get_ext_rcode(const knot_pkt_t *pkt);