productGroups selector withIds retuns an error

Question

On adWords scripts, I have a query like this:

https://developers.google.com/adwords/scripts/docs/reference/adwordsapp/adwordsapp_productgroupselector#withIds_1

function seleccionarHijosFeed(id) {

    var hijo = AdWordsApp
     .productGroups()
     .withIds([id]).get();

    while (hijo.hasNext()) {
        return hijo.next();
    }
    return 'undefined';
}

It must return a ProductGroup, but it Gives me an error:

Each id must be a 2-element array.

As in some reports you need to give also de AdGroupId, I've tried changing the query:

function seleccionarHijosFeed(id, adGroupId) {

    var hijo = AdWordsApp
     .productGroups()
     .withIds([adGroupId, id]).get();

    while (hijo.hasNext()) {
        return hijo.next();
    }
    return 'undefined';
}

But the error log appears again:

Each id must be a 2-element array.

The Ids are ok, I've done a log.

Coul anyone help me?