public function setActionError($key, $message) {
        if (!isset($message)) {
            unset($this->attributes[ACTION_ERRORS_KEY][$key]);
        }
        elseif (is_string($message)) {
            $this->attributes[ACTION_ERRORS_KEY][$key] = $message;
        }
        else throw new IllegalTypeException('Illegal type of parameter $message: '.gettype($message));
    }