Null fields in SFDC via SOAP

NullField.xml

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:urn="urn:enterprise.soap.sforce.com" xmlns:urn1="urn:sobject.enterprise.soap.sforce.com">
   <soapenv:Header>
      <urn:SessionHeader>
         <urn:sessionId>${#Project#sessionid}</urn:sessionId>
      </urn:SessionHeader>
   </soapenv:Header>
   <soapenv:Body>
      <urn:update>
         <!--Zero or more repetitions:-->
         <urn:sObjects xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="Contact">
               <urn:Id>${#Project#contactid}</urn:Id>
               <urn:fieldsToNull>CustomField__c</urn:fieldsToNull>
         </urn:sObjects>
      </urn:update>
   </soapenv:Body>
</soapenv:Envelope>

UpdateField.xml

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:urn="urn:enterprise.soap.sforce.com" xmlns:urn1="urn:sobject.enterprise.soap.sforce.com">
   <soapenv:Header>
      <urn:SessionHeader>
         <urn:sessionId>${#Project#sessionid}</urn:sessionId>
      </urn:SessionHeader>
   </soapenv:Header>
   <soapenv:Body>
      <urn:update>
         <!--Zero or more repetitions:-->
         <urn:sObjects xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="Contact">
               <urn:Id>${#Project#contactid}</urn:Id>
               <urn:CustomField__c>Yay! Data</urn:CustomField__c>
         </urn:sObjects>
      </urn:update>
   </soapenv:Body>
</soapenv:Envelope>