diff --git a/src/wp-includes/user.php b/src/wp-includes/user.php
index 9a60cd2069..a0cb8d815f 100644
--- a/src/wp-includes/user.php
+++ b/src/wp-includes/user.php
@@ -212,6 +212,8 @@ function count_many_users_posts( $users, $post_type = 'post', $public_only = fal
  * @return int The current user's ID
  */
 function get_current_user_id() {
+	if ( ! function_exists( 'wp_get_current_user' ) )
+		return 0;
 	$user = wp_get_current_user();
 	return ( isset( $user->ID ) ? (int) $user->ID : 0 );
 }