diff --git a/src/wp-admin/includes/class-core-upgrader.php b/src/wp-admin/includes/class-core-upgrader.php
index bda7901af3..dd3f540dda 100644
--- a/src/wp-admin/includes/class-core-upgrader.php
+++ b/src/wp-admin/includes/class-core-upgrader.php
@@ -57,7 +57,7 @@ class Core_Upgrader extends WP_Upgrader {
 	 *        @type bool $do_rollback      Whether to perform this "upgrade" as a rollback.
 	 *                                     Default false.
 	 * }
-	 * @return null|false|WP_Error Null on success, false or WP_Error on failure.
+	 * @return string|false|WP_Error New WordPress version on success, false or WP_Error on failure.
 	 */
 	public function upgrade( $current, $args = array() ) {
 		global $wp_filesystem;
diff --git a/src/wp-admin/includes/update-core.php b/src/wp-admin/includes/update-core.php
index 0f6f072243..48e3e1277b 100644
--- a/src/wp-admin/includes/update-core.php
+++ b/src/wp-admin/includes/update-core.php
@@ -880,7 +880,7 @@ $_new_bundled_files = array(
  *
  * @param string $from New release unzipped path.
  * @param string $to   Path to old WordPress installation.
- * @return null|WP_Error Null on success, WP_Error on failure.
+ * @return string|WP_Error New WordPress version on success, WP_Error on failure.
  */
 function update_core( $from, $to ) {
 	global $wp_filesystem, $_old_files, $_new_bundled_files, $wpdb;