Call wp_generate_password() from within a Class

I have a function to add wp users. There is a $password = wp_generate_password(); in it. This works.

Now I'm creating a class XenWord_Add_WP_User based on the function. In fact, I copied the function to become a method public function xenword_add_wp_user()

However, this is failing. The error states call to undefined function wp_generate_password(). How can I call wp_generate_password() within a class?

Here is a portion of the class.

Class XenWord_Add_WP_User {

public function xenword_add_wp_user() {
    /**
     * @since 1.0.6
     * Insert the current user into the wp_users and wp_usermeta table
     *
     * @since 1.0.7
     * Moved this code to a separate function.
     *
     * TODO: This code needs to be cleaned up
     */

    global $wpdb;

    $xenword_options = get_option( 'xenword_options' );

    $visitor = XenForo_Visitor::getInstance();

    $user_ID = $visitor->get( 'user_id' );

    /* Added 2.0.1.02 */
    $wp_default_role = $xenword_options['wp_default_role'];

    $user_check = $wpdb->get_var( "SELECT COUNT(*) FROM $wpdb->users WHERE ID = '$user_ID' " );
    $user_id  = $visitor->get( 'user_id' );
    $password = wp_generate_password();
    $nickname = $visitor->get( 'username' );
    $email    = $visitor->get( 'email' );
    $current_registration = date('Y/m/d H:i:s', $visitor->get('register_date'));
// removed the rest of the class in this post since the failure is a few lines up.
    }
}
$wp_add_user = new XenWord_Add_WP_User();