diff --git a/src/wp-includes/rewrite.php b/src/wp-includes/rewrite.php
index 647977cc2041e..235c0c82e25f6 100644
--- a/src/wp-includes/rewrite.php
+++ b/src/wp-includes/rewrite.php
@@ -244,7 +244,7 @@ function remove_permastruct( $name ) {
  *
  * @global WP_Rewrite $wp_rewrite WordPress rewrite component.
  *
- * @param string   $feedname Feed name.
+ * @param string   $feedname Feed name. Should not start with '_'.
  * @param callable $callback Callback to run on feed display.
  * @return string Feed action name.
  */