diff --git a/lib/index.js b/lib/index.js
index 54afc88..1744c83 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -44,7 +44,7 @@ function hmac(input, key) {
  *
  * @param {KeypairPrivateSlice} keypair
  * @param {B4A} msg
- * @param {B4A | string | undefined} hmacKey
+ * @param {(B4A | string)=} hmacKey
  * @returns {string}
  */
 function sign(keypair, msg, hmacKey) {
@@ -64,7 +64,7 @@ function sign(keypair, msg, hmacKey) {
  * @param {KeypairPublicSlice} keypair
  * @param {B4A} msg
  * @param {string} sig
- * @param {B4A | string | undefined} hmacKey
+ * @param {(B4A | string)=} hmacKey
  * @returns {boolean}
  */
 function verify(keypair, msg, sig, hmacKey) {