PO from Pending Bill - Billed

Hi all,
   we are using Webservices to add a vendor bill associated with a PO. We use InitializeRecord but we are facing problem and the NS doesn't associate the bill with the PO... please take a look of our code and let us know if can you help us...

   NetSuiteBindingStub stub = (NetSuiteBindingStub) port;
   stub.clearHeaders();
   SOAPHeaderElement prefHeader = new SOAPHeaderElement("urn:messages_2_6.platform.webservices.netsuite.com", "preferences");
   Preferences preferences = new Preferences();
   preferences.setIgnoreReadOnlyFields(Boolean.TRUE);
   prefHeader.setObjectValue(preferences);
   stub.setHeader(prefHeader);
  
   InitializeRecord initializeRecord = new InitializeRecord();
  
   InitializeRef po = new InitializeRef();
   po.setInternalId(poID);
   po.setType(InitializeRefType.purchaseOrder);
  
   initializeRecord.setType(InitializeType.vendorBill);
   initializeRecord.setReference(po);