Ticket #18876: 18876.patch

wp-includes/update.php

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