public function addError($error) {
        // Make sure that we don't try to index an error that doesn't
        // contain an index value.
        if ($error->getErrorCode() == null) {
            require_once 'Zend/Gdata/App/Exception.php';
            throw new Zend_Gdata_App_Exception("Error encountered without corresponding error code.");
        }

        $this->_errors[$error->getErrorCode()] = $error;
    }