Index: wp-admin/admin-header.php
===================================================================
--- wp-admin/admin-header.php	(revision 50941)
+++ wp-admin/admin-header.php	(working copy)
@@ -35,6 +35,15 @@
 $title = strip_tags( $title );
 
 if ( is_network_admin() ) {
+
+    /*
+     * If multisite is not enabled, the get_network function is not available.
+     * We perform a multisite check to prevent a fatal error.
+     */
+    if ( ! is_multisite() ) {
+        wp_redirect( admin_url() );
+        exit;
+    }
 	/* translators: Network admin screen title. %s: Network title. */
 	$admin_title = sprintf( __( 'Network Admin: %s' ), get_network()->site_name );
 } elseif ( is_user_admin() ) {