diff --git a/src/wp-includes/class-walker-category-dropdown.php b/src/wp-includes/class-walker-category-dropdown.php
index 46821fbc62..3b36258af8 100644
--- a/src/wp-includes/class-walker-category-dropdown.php
+++ b/src/wp-includes/class-walker-category-dropdown.php
@@ -41,6 +41,7 @@ class Walker_CategoryDropdown extends Walker {
 	 * @param int    $depth    Depth of category. Used for padding.
 	 * @param array  $args     Uses 'selected', 'show_count', and 'value_field' keys, if they exist.
 	 *                         See {@see wp_dropdown_categories()}.
+	 * @param int    $id       Optional. ID of the current category. Default 0 (unused).
 	 */
 	public function start_el( &$output, $category, $depth = 0, $args = array(), $id = 0 ) {
 		$pad = str_repeat(' ', $depth * 3);