尝试使用Zend_Session_Namespace设置会话变量，每次都为NULL

I'm running an application where in the controller I'm trying to set session variables using Zend_Session and Zend_Session_Namespace:

Bootstrap.php

protected function _initSession()
{
    Zend_Session::start();
}

SomeController.php

protected function updateQuestionViewsTotal($question)
{
    $userSession = new Zend_Session_Namespace('QA_Session');

    if (! is_array($userSession->questionViews)) {
        $userSession->questionViews = array();
    }

    // create session array to contain the questions this
    // user has viewed.
    if(array_search($question->id, $userSession->questionViews) === false) {
        $question->views_total++;
        $question->save();
    }

    // ensure that this page is in the array
    array_push($userSession->questionViews, $question->id);
    $userSession->questionViews = array_unique($userSession->questionViews);
}

As you can see from above, I have within one of my controllers a method with an attempt to use session variables via Zend_Session_Namespace.

However, when I insert a var_dump on the second page load (refresh):

protected function updateQuestionViewsTotal($question)
{
    $userSession = new Zend_Session_Namespace('QA_Session');
    var_dump($userSession));
    if (! is_array($userSession->questionViews)) {
        $userSession->questionViews = array();
    }

..Please note: this is AFTER I've run it once, so I'm expecting that the session variable has been set. Anyway on every occasion, it is NULL. So it would seem that the variable isn't being written to $_SESSION? What am I doing wrong?