I am trying to run the following code in a simple sharepoint app, but I got this error:

Uncaught Error: The property or field has not been initialized. It has not been requested or the request has not been executed. It may need to be explicitly requested. 

Code is this one:

var collListItems;

$(document).ready(function () {

    getConfigValues();

});

function getConfigValues() {

    var context = SP.ClientContext.get_current();

    var configList = context.get_web().get_lists().getByTitle('Configuration Values');

    var camlQuery = new SP.CamlQuery();

    collListItems = configList.getItems(camlQuery);

    context.load(collListItems);

    context.executeQueryAsync(onGetConfigValuesSuccess, onGetConfigValuesFail);

}

function onGetConfigValuesSuccess() {

    var OrgLogoUrl;

    var OrgName;

    var listItemEnumerator = collListItems.getEnumerator();

    while (listItemEnumerator.moveNext()) {

        var oListItem = listItemEnumerator.get_current();

        var current = oListItem.get_item('Title');

        switch (current) {

            case 'OrganizationName':

                OrgName = oListItem.get_item('Value');

                break;

            case 'OrganizationLogoUrl':

                OrgLogoUrl = oListItem.get_item('Value');

                break;

        };

    }

    if (OrgName && OrgName.length > 0) {

        $('#DeltaPlaceHolderPageTitleInTitleArea').html(OrgName);

        $('.ms-siteicon-img').attr('title', OrgName);

    }

    if (OrgLogoUrl && OrgLogoUrl.length > 0)

        $('.ms-siteicon-img').attr('src', OrgLogoUrl);

    else

        $('.ms-siteicon-img').attr('src', '../Images/AppLogo.png');

}

function onGetConfigValuesFail(sender, args) {

    alert('Failed to get the Configuration Values. Error:' + args.get_message());

}

The code is from a book, without any modifications:

OrgName = oListItem.get_item('Value');