Index: wp-includes/load.php
===================================================================
--- wp-includes/load.php	(revision 17079)
+++ wp-includes/load.php	(working copy)
@@ -610,11 +610,6 @@
  * @return bool True if inside WordPress network administration pages.
  */
 function is_network_admin() {
-	if ( defined('DOING_AJAX') && function_exists('get_current_screen') ) {
-		$screen = get_current_screen();
-		return (bool) $screen->is_network;
-	}
-
 	if ( defined( 'WP_NETWORK_ADMIN' ) )
 		return WP_NETWORK_ADMIN;
 	return false;
Index: wp-admin/admin-ajax.php
===================================================================
--- wp-admin/admin-ajax.php	(revision 17079)
+++ wp-admin/admin-ajax.php	(working copy)
@@ -57,6 +57,9 @@
 	$current_screen->is_network = 'false' === $current_screen->is_network ? false : true;
 	$current_screen->is_user = 'false' === $current_screen->is_user ? false : true;
 
+	define( 'WP_NETWORK_ADMIN', $current_screen->is_network );
+	define( 'WP_USER_ADMIN', $current_screen->is_user );
+
 	$wp_list_table = get_list_table( $_GET['list_args']['class'] );
 	if ( ! $wp_list_table )
 		die( '0' );