diff --git a/src/wp-includes/post.php b/src/wp-includes/post.php
index deb215e17d..f621f456ba 100644
--- a/src/wp-includes/post.php
+++ b/src/wp-includes/post.php
@@ -6821,10 +6821,10 @@ function _publish_post_hook( $post_id ) {
  *
  * @since 3.1.0
  *
- * @param int|WP_Post $post Post ID or post object. Defaults to global $post.
+ * @param int|WP_Post|null $post Optional. Post ID or post object. Defaults to global $post.
  * @return int|false Post parent ID (which can be 0 if there is no parent), or false if the post does not exist.
  */
-function wp_get_post_parent_id( $post ) {
+function wp_get_post_parent_id( $post = null ) {
 	$post = get_post( $post );
 	if ( ! $post || is_wp_error( $post ) ) {
 		return false;