What should I pass back on a Custom Laravel Validation fail?

gistfile1.php

    /**
     * Check the user id is the Auth'd user id
     *
     * @param string $attribute
     * @param mixed $value
     * @return bool
     */
    protected function validateUserIdIsAuthUserId( $attribute, $value )
    {
        if($value===Auth::user()->id)
        {
            return $value;
        }

        return false; // ? what should I return on fail?
    }