Custom Field is not supported for object type {!{Work Order}!}

I'm trying to set a new custom field for a work order, however it doesn't look like it's allowing me for this one. I am able to successfully set a different custom field with Id of 4 (WORK ORDER CUSTOM FIELDS) without issue but looks like I'm not able to do so with custom field Id of 12 (CUSTOMER CUSTOM FIELDS), what might I be missing?

thanks,
Raul

REQUEST

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:int="http://corrigo.com/integration/">
soap:Header
int:CorrigoNetOptions
int:ImposeConcurrencyIdfalse</int:ImposeConcurrencyId>
int:UpdateLastModifiedfalse</int:UpdateLastModified>
int:CanDeleteMissingEntityfalse</int:CanDeleteMissingEntity>
int:LockOnDataRetrievalDefault</int:LockOnDataRetrieval>
</int:CorrigoNetOptions>
</soap:Header>
soap:Body
int:Execute
<int:commandRequest xsi:type="int:UpdateCommand" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<int:Entity xsi:type="int:WorkOrder">
int:Id134786</int:Id>
int:CustomFields
int:CustomField2

int:Descriptor
int:Id12</int:Id>
</int:Descriptor>
int:ValueOKC</int:Value>
</int:CustomField2>
</int:CustomFields>
</int:Entity>
<int:PropertySet xsi:type="int:PropertySet">
int:Properties
int:stringId</int:string>

int:stringCustomFields.Descriptor.Id</int:string>
int:stringCustomFields.Value</int:string>
</int:Properties>
</int:PropertySet>
</int:commandRequest>
</int:Execute>
</soap:Body>
</soap:Envelope>

RESPONSE

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
soap:Body

</soap:Body>
</soap:Envelope>