Ticket #18715: update-patch.diff

wp-includes/update.php

@@ -395 +395 @@
                 wp_schedule_event(time(), 'twicedaily', 'wp_update_themes');
 }
 
-if ( ! is_main_site() )
+if ( !function_exists( 'is_main_site' ) || ! is_main_site() )
         return;
 
 add_action( 'admin_init', '_maybe_update_core' );