Index: wp-includes/update.php
===================================================================
--- wp-includes/update.php	(revision 18731)
+++ wp-includes/update.php	(working copy)
@@ -395,7 +395,7 @@
 		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' );