Search for Cash Sale record by createdFrom value

I need to search for a Cash Sale Record using the internalId of the Sales Order record that it was created from. Apparently my searchValue is not valid:

function getCashSaleIntId($CreatedBySOIntId) {
        $service = new NetSuiteService();
        $service->setSearchPreferences(false, 20);

        $ssField = new SearchStringField();
        $ssField->operator = "is";
        $ssField->searchValue = $CreatedBySOIntId;
        $search = new TransactionSearchBasic();
        $search->createdFrom->internalId = $ssField;
        $request = new SearchRequest();
        $request->searchRecord = $search;
        $searchResponse = $service->search($request);
        return $searchRepsonse->internalId;
}

results:
SearchResponse Object
(
    [searchResult] => SearchResult Object
        (
            [status] => Status Object
                (
                    [statusDetail] => Array
                        (
                            [0] => StatusDetail Object
                                (
                                    [code] => INVALID_SEARCH_VALUE
                                    [message] => You need to provide a search value.