error on isValid() in tests

[garak]

Given this simple test:

<?php
include dirname(__FILE__).'/../../bootstrap/functional.php';
$browser = new sfTestFunctional(new sfBrowser());
$browser->
  info('about page')->
  get('/about')->
  with('request')->begin()->
    isParameter('module', 'about')->
    isParameter('action', 'index')->
  end()->
  with('response')->begin()->
    isStatusCode(200)->
    checkElement('title', '/About/')->
    isValid(true)->
  end()
;

I'm getting the following errors

failed to load external entity "file:/[project]/cache/sf_tester_response/w3/TR/xhtml1/DTD/xhtml-symbol.ent"`
failed to load external entity "file:/[project]/cache/sf_tester_response/w3/TR/xhtml1/DTD/xhtml-special.ent"`

when the test is trying the isValid(true)

I checked the path, the files mentioned in errors are there