Error while running the Recommender

On the Recommender API page, I chose to run the Recommender with the history_rec module, and I got the error below.

An error occurred. /batch?id=10&op=do <br /> <b>Fatal error</b>: Call to private Matrix::__construct() from context 'Matrix' in <b>/work/projects/testsites/nss1/www/sites/all/modules/contrib/recommender/Matrix.php</b> on line <b>32</b><br /> 

I'm using PHP v 5.2.5, and Drupal 6.