Index: wp-includes/load.php
===================================================================
--- wp-includes/load.php	(revision 17079)
+++ wp-includes/load.php	(working copy)
@@ -612,7 +612,9 @@
 function is_network_admin() {
 	if ( defined('DOING_AJAX') && function_exists('get_current_screen') ) {
 		$screen = get_current_screen();
-		return (bool) $screen->is_network;
+		if ( isset( $screen->is_network ) )
+			return (bool) $screen->is_network;
+		return false;
 	}
 
 	if ( defined( 'WP_NETWORK_ADMIN' ) )