public function valueKeys()
    {
        if (!$this->isValid()) {
            throw new \LogicException('Session is in an invalid state.');
        }

        return array_values(
                array_filter(
                        $this->storage->valueKeys(),
                        function($valueKey)
                        {
                            return substr($valueKey, 0, 11) !== '__stubbles_';
                        }
                )
        );
    }