Set Object Field to NULL in SOAP Request

Hi,

I have been at this for sometime but can't seem to figure out how to set a field in an object reference to null in a soap request. Kindly see an example soap below:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:adm="http://my/namespace/">
   <soapenv:Body>
      <adm:WebMethod>
         <adm:reference>
            <adm:Field1></adm:Field1>
            <adm:ModelName>test</adm:ModelName>
            <adm:DateAdded></adm:DateAdded>
            <adm:DateModified></adm:DateModified>
         </adm:reference>
      </adm:WebMethod>
   </soapenv:Body>
</soapenv:Envelope>

The above throws an exception:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
   <soap:Body>
      <soap:Fault>
         <faultcode>soap:Client</faultcode>
         <faultstring>System.Web.Services.Protocols.SoapException: Server was unable to read request. ---> System.InvalidOperationException: There is an error in XML document (14, 52). ---> System.FormatException: Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx).
   at System.Guid..ctor(String g)
   at System.Xml.XmlConvert.ToGuid(String s)
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReader1.Read4_NullableOfGuid(Boolean checkType)
...
</faultstring>
         <detail/>
      </soap:Fault>
   </soap:Body>
</soap:Envelope>

I have tried leaving the '?' or an empty tag and it still does not work.

Cheers
Henry