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:ImposeConcurrencyId>false</int:ImposeConcurrencyId>
<int:UpdateLastModified>false</int:UpdateLastModified>
<int:CanDeleteMissingEntity>false</int:CanDeleteMissingEntity>
<int:LockOnDataRetrieval>Default</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:Id>134786</int:Id>
<int:CustomFields>
<int:CustomField2>

<int:Descriptor>
<int:Id>12</int:Id>
</int:Descriptor>
<int:Value>OKC</int:Value>
</int:CustomField2>
</int:CustomFields>
</int:Entity>
<int:PropertySet xsi:type="int:PropertySet">
<int:Properties>
<int:string>Id</int:string>

<int:string>CustomFields.Descriptor.Id</int:string>
<int:string>CustomFields.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>

0
Custom Field is not supported for object type {!{Work Order}!}
Corrigo.Core.CorrigoCoreException

WorkOrder
134786
0

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