Amazon Product API undefined result

I am using following code get results from AWS product advertising api in node.js.

    var util = require('util'),
        OperationHelper = require('apac').OperationHelper;

    var opHelper = new OperationHelper({
        awsId:     '%myid%',
        awsSecret: '%mysecret%',
        assocId:   '%myassociate%',
        version:   '2013-08-01'
    });

    opHelper.execute('ItemSearch', {
      'SearchIndex': 'All',
      'Keywords': 'harry potter'
    }, function(err, results) {
        console.log(results);
    });

Result: undefined