diff --git a/writeToStream.js b/writeToStream.js
index 448726a..a722ce9 100644
--- a/writeToStream.js
+++ b/writeToStream.js
@@ -201,6 +201,8 @@ function connect (packet, stream, opts) {
   // Password
   if (password != null) {
     if (!providedUsername && protocolVersion !== 5) {
+      // `username` is not required when password is present in MQTT-v5
+      // see also: https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901044
       stream.destroy(new Error('Username is required to use password'))
       return false
     }