Index: src/wp-admin/includes/class-wp-upgrader.php
===================================================================
--- src/wp-admin/includes/class-wp-upgrader.php	(revision 29511)
+++ src/wp-admin/includes/class-wp-upgrader.php	(working copy)
@@ -1045,6 +1045,7 @@
 				'destination' => get_theme_root( $theme ),
 				'clear_destination' => true,
 				'clear_working' => true,
+				'is_multi' => true,
 				'hook_extra' => array(
 					'theme' => $theme
 				),