Updating CustomField with GenericObjects

Content

I am trying to update CustomFields of a Contact object using GenericObjects, but I get an error from the server.

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<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"&gt;
    <soapenv:Header></soapenv:Header>
    <soapenv:Body>
        <soapenv:Fault>
            <faultcode>soapenv:Sender</faultcode>
            <faultstring>Mismatched classes - GenericObject is declared as ContactCustomFields but is expected to be GenericObject
While trying to populate Contact.CustomFields of type GenericObject
While translating GenericObject to Contact processing GenericField at index 0
While trying to process GenericObject as Contact</faultstring>
            <detail>
                <n0:RequestErrorFault xmlns:n0="urn:faults.ws.rightnow.com/v1_2"
                    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"&gt