Index: src/wp-includes/user.php
===================================================================
--- src/wp-includes/user.php	(revision 25907)
+++ src/wp-includes/user.php	(working copy)
@@ -212,7 +212,7 @@
  * @return int The current user's ID
  */
 function get_current_user_id() {
-	$user = wp_get_current_user();
+	$user = function_exists( 'wp_get_current_user' ) ? wp_get_current_user() : new WP_User;
 	return ( isset( $user->ID ) ? (int) $user->ID : 0 );
 }