The following is the SOAP XML I am attempting to send:

<?xml version="1.0"?>
<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:Header>
  <SessionHeader xmlns="urn:partner.soap.sforce.com">
   <sessionId>rZqNaNMO_o2MKBuw1xwkAPpZemaKvLZD0iu5bafkjUpTVAbfklPDMxhst5paaGcGHMMCIkYJU3INBGFPSBoPCvbtzKggVlhL</sessionId>
  </SessionHeader>
 </soap:Header>
 <soap:Body>
  <create xmlns="urn:partner.soap.sforce.com">
   <sObjects>
    <type xmlns="urn:sobject.partner.soap.sforce.com">Opportunity</type>
    <Id xsi:nil="true" xmlns="urn:sobject.partner.soap.sforce.com"/>
    <Name xmlns="">test opp</Name>
    <Product__c xmlns="">Celotex</Product__c>
    <Description xmlns="">test</Description>
    <Amount xmlns="">0</Amount>
    <AccountID xmlns="">00120000000lVS0AAM</AccountID>
    <CloseDate xmlns="">2004-06-15</CloseDate>
   </sObjects>
  </create>
 </soap:Body>
</soap:Envelope>
 

which returns:

<?xml version="1.0"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
 <soapenv:Body>
  <soapenv:Fault>
   <faultcode>soapenv:Server.userException</faultcode>
   <faultstring>common.exception.ApiException: Must send a concrete entity type.</faultstring>
   <detail>
    <sf:fault xsi:type="sf:InvalidSObjectFault" xmlns:sf="urn:fault.enterprise.soap.sforce.com">
     <sf:exceptionCode>INVALID_TYPE</sf:exceptionCode>
     <sf:exceptionMessage>Must send a concrete entity type.</sf:exceptionMessage>
    </sf:fault>
   </detail>
  </soapenv:Fault>
 </soapenv:Body>
</soapenv:Envelope>

What could be causing this?  I don't see anything wrong with the SOAP.  I am using a JavaScript-based IDE so I'm basically just working with raw SOAP.

thanks