Zend_Session: Session must be started before any output has been sent to the browser

I've run into this issue before, but I can't remember how to solve it. I have created a bare bones (can't get any simpler) controller, and am just trying to echo something to the browser, and I am getting this message:

Fatal error: Uncaught exception 'Zend_Session_Exception' with message 'Session must be started before any output has been sent to the browser ...

Here's my entire controller. It's displaying 'success', but it also displays the error message. How can I silence that error message so I can simply echo something to the browser?

<?php

class CacheController extends Zend_Controller_Action
{
    public function clearAction()
    {
        $this->_helper->layout->disableLayout();
        $this->_helper->viewRenderer->setNoRender();
        try {
            $result = Model_Cache::emptyCache(array('foobar'=>1));

            if ($result['status'] == true) {
                echo 'Success';
            } else {
                echo 'Error: ' . $result['message'];
            }
        } catch (Exception $e) {
            echo 'Error: ' . $e->getMessage();
        }
    }
}

For some reason, I was trying to set some session variable in my index.php file after the application had finished bootstrapping. Not sure why this didn't cause a problem before, but when I removed this from my index.php, it worked.

$userSession = new Zend_Session_Namespace('Auth');
$userSession->forcePasswordChange = false;