add_action doesn’t work for my function

HI please check below code.

add_action( 'user_register', 'fill_identity', 10, 1 );
function fill_identity($user_id){

    $current_user = get_userdata($user_id);
    $mail_user = $current_user->user_email;
    $mail_user = substr($mail_user,0,-14);
    $mail_user = explode(".", $mail_user);
    $prenom = $mail_user[0] = ucfirst($mail_user[0]);
    $nom = $mail_user[1] = ucfirst($mail_user[1]);
    var_dump($mail_user);
    wp_update_user(array(
        'ID' => $current_user->ID, 
        'first_name' => $prenom,
        'last_name' => $nom,
    ));
}