diff --git a/library/ssl_tls13_keys.c b/library/ssl_tls13_keys.c
index 8725d1495..ec2fe3b23 100644
--- a/library/ssl_tls13_keys.c
+++ b/library/ssl_tls13_keys.c
@@ -62,6 +62,8 @@ struct mbedtls_ssl_tls1_3_labels_struct const mbedtls_ssl_tls1_3_labels =
  *                  The label length MUST be
  *                  <= MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_LABEL_LEN
  *                  It is the caller's responsiblity to ensure this.
+ *                  All (label, label length) pairs used in TLS 1.3
+ *                  can be obtained via MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN().
  * - (ctx, clen): context + context length
  *                The context length MUST be
  *                <= MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_CONTEXT_LEN