Update Custom Field for sales order

I can't seem to update a custom field for a specific sales order using the PHP api.

I'm getting a response of: INVALID_KEY_OR_REF, The specified key is invalid.

2074 is the order # I'm trying to edit and 229 is the internal id of the custom transaction body field I made. Any ideas?
[CODE]<?php
    
    require_once '../PHPToolkit/NetSuiteService.php';
    
    $service = new NetSuiteService();
    
    $so = new SalesOrder();
    $so->internalid = 2074;
    $so->entity = new RecordRef();
    
    $customFieldList = new CustomFieldList();
    $customField = new StringCustomFieldRef();
    $customField->value = utf8_encode("12345");
    $customField->internalId = '229';
    $customFieldList->customField[] = $customField;
    
    $so->customFieldList = $customFieldList;