Retrieving VendorBill from Transaction Search

I want to retrieve vendorBills from Netsuite.

Here is my code

TransactionSearch transactionSearch = new TransactionSearch();

            TransactionSearchBasic tranSearchBasic = new TransactionSearchBasic();
            TransactionSearchAdvanced tranAdvancedSearch = new TransactionSearchAdvanced();

            SearchDateField searchDate = new SearchDateField();
            searchDate.setOperator(SearchDateFieldOperator.within);
            searchDate.setSearchValue(startDate);
            searchDate.setSearchValue2(endDate);
            tranSearchBasic.setDateCreated(searchDate);
transactionSearch.setBasic(tranSearchBasic);

            tranAdvancedSearch.setCriteria(transactionSearch);

            SearchResult billResult = port.search(tranAdvancedSearch);
The above code retruns all Transaction records created between specified date. But I want to filter out only vendorBill.

I have tried the following
String[] type = new String[1];
type[0]=RecordType._vendorBill;
tranSearchBasic.setType(type);

but it returns null.

webservice host address:https://webservices.na1.netsuite.com/wsdl/v2012_2_0/netsuite.wsdl