Custom Field of a Custom Record update issue

Hello!

Now we are trying to update several custom fields of a custom record.  We have been able to add the custom records no problem but getting a "java.lang.NullPointerException" every time we try to update the record in SuiteTalk and not sure what is wrong or what is null. 

Here is our code for updating

            CustomRecord pbMaster = new CustomRecord();
            pbMaster.internalId = "79";

            StringCustomFieldRef stringCustomFieldRef = new StringCustomFieldRef();
            stringCustomFieldRef.scriptId = "custrecord_pe_pbmastertrackno";
            stringCustomFieldRef.value = txtMasterTrackno.Text.Trim();

            CustomFieldRef[] customFieldRef = new CustomFieldRef[1];
            customFieldRef[0] = stringCustomFieldRef;

            pbMaster.customFieldList = customFieldRef;

            _service = new NetSuiteService();

            setPassport();      // Set the Passport information for authentication
            WriteResponse writeResponse = _service.update(pbMaster);