Index: wp-includes/update.php
===================================================================
--- wp-includes/update.php	(revision 18519)
+++ wp-includes/update.php	(working copy)
@@ -385,7 +385,7 @@
 		wp_schedule_event(time(), 'twicedaily', 'wp_update_themes');
 }
 
-if ( ! is_main_site() )
+if ( ! is_main_site() || ( defined( 'DOING_AJAX' ) && DOING_AJAX ) )
 	return;
 
 add_action( 'admin_init', '_maybe_update_core' );