[fw-general] how to use json action context?

[Tim]

I can't seem to get my controllers to recognize that I only want them
to display json. Here's my controller

<?php

class IndexController extends Zend_Controller_Action {
        public function init() {
                $this->_helper->contextSwitch()
                        ->clearContexts()
                        ->clearActionContexts()
                        ->setDefaultContext('json')
                        ->initContext('json');
        }

        public function indexAction() {
                $this->view->assign(array(
                        'status' => 'ok',
                        'message' => 'This is the index action'
                ));
        }
}

?>


All I want to do is, by default, output everything as json. I don't
want to have to send a "format=json" parameter in the URL.

According to 
http://framework.zend.com/manual/en/zend.controller.actionhelpers.html
I should be able to do this, but the above controller results in the
error

Fatal error: Uncaught exception 'Zend_View_Exception' with message
'script 'error/error.phtml' not found in path

What am I missing?

Thanks,
Tim