Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 36172)
+++ src/wp-includes/theme.php	(working copy)
@@ -746,8 +746,9 @@
 	 *
 	 * @param string   $new_name  Name of the new theme.
 	 * @param WP_Theme $new_theme WP_Theme instance of the new theme.
+	 * @param WP_Theme $old_theme WP_Theme instance of the old theme.
 	 */
-	do_action( 'switch_theme', $new_name, $new_theme );
+	do_action( 'switch_theme', $new_name, $new_theme, $old_theme );
 }
 
 /**