diff --git a/src/wp-includes/user.php b/src/wp-includes/user.php
index d0b760986c..8bc44ae233 100644
--- a/src/wp-includes/user.php
+++ b/src/wp-includes/user.php
@@ -1005,12 +1005,12 @@ function count_users( $strategy = 'time', $site_id = null ) {
  * @global string  $user_url      The url in the user's profile
  * @global string  $user_identity The display name of the user
  *
- * @param int $for_user_id Optional. User ID to set up global data.
+ * @param int $for_user_id Optional. Default 0. User ID to set up global data.
  */
-function setup_userdata( $for_user_id = '' ) {
+function setup_userdata( $for_user_id = 0 ) {
 	global $user_login, $userdata, $user_level, $user_ID, $user_email, $user_url, $user_identity;
 
-	if ( '' == $for_user_id ) {
+	if ( ! $for_user_id ) {
 		$for_user_id = get_current_user_id();
 	}
 	$user = get_userdata( $for_user_id );