Update Custom Object Fields - java.lang.NullPointerException

I have a helper method for updating a single field of a custom object, but when I run it I get an error: java.lang.NullPointerException

As far as I can tell I am following the "update" example in SuiteTalkWebServicesGuide. Can anyone offer some insight as to where I went wrong?

 private static void UpdateCustomRecordValue(CustomRecord customRecord, string fieldName, string newValue, NetSuiteService netsuiteService)  {      var field = customRecord.customFieldList.Where(custField => custField.scriptId == fieldName).FirstOrDefault();      if (field == null) return;        var updateRecord = new CustomRecord();      updateRecord.scriptId = customRecord.scriptId;        CustomFieldRef[] custFieldList = new CustomFieldRef[] {          new StringCustomFieldRef          {              value = newValue,              scriptId = field.scriptId          }      };        updateRecord.customFieldList = custFieldList;      var updateResponse = netsuiteService.update(updateRecord);    }