diff --git a/wp-includes/post.php b/wp-includes/post.php
index 286ad8ccf6..5a0671ac46 100644
--- a/wp-includes/post.php
+++ b/wp-includes/post.php
@@ -7642,12 +7642,13 @@ function _publish_post_hook( $post_id ) {
  * Returns the ID of the post's parent.
  *
  * @since 3.1.0
+ * @since 5.9.0 The `$post` parameter was made optional.
  *
- * @param int|WP_Post $post Post ID or post object.
+ * @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;
diff --git a/wp-includes/version.php b/wp-includes/version.php
index ee990fb251..e49aadf12b 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52193';
+$wp_version = '5.9-alpha-52194';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.