eZpublish Search Service Problems

gistfile1.php

<?php
        $oSearchService = $this->getRepository()->getSearchService();

        $aCriteria = array();
        $aCriteria[] = new Criterion\ContentTypeIdentifier('item_class');
        $aCriteria[] = new Criterion\Field( 'description', '=', 'Value of description field' );

        $oQuery = new Query();
        $oQuery->criterion = new Criterion\LogicalAnd($aCriteria);
        $oQuery->limit = 20;

        return $oSearchService->findContent($oQuery);