[fw-mvc] PHPUnit testing generating Zend_Dom_Exception

I'm getting up and running with PHPUnit after following the video
tutorial and other resources at http://www.zendcasts.com/unit-testing-with-the-zend-framework-with-zend_test-and-phpunit/

However, when using the methods assertQueryContentContains() or
assertXpathContentContains(), I get the following exception:

Zend_Dom_Exception: Error parsing document (type == docXml)

Other methods such as assertModule(), assertController(),
assertAction() and assertResponseCode() are working fine.

Has anyone else encountered this, or have clues as to how to fix?

Regards,

Ian