MAPPING.EXCEPTION_DURING_EXECUTE

Hi Gurus,

I am working on SOAP to Proxy Scenario in PI7.3,

Following is the error messge I am getting while testing with SAOP UI

<SOAP:Body>

      <SOAP:Fault>

         <faultcode>SOAP:Server</faultcode>

         <faultstring>System Error</faultstring>

         <detail>

            <s:SystemError xmlns:s="http://sap.com/xi/WebService/xi2.0">

               <context/>

               <code>MAPPING.EXCEPTION_DURING_EXECUTE</code>

               <text>Runtime exception occurred during application mapping com/sap/xi/tf/_MM_Autopurchaseorder_IB_; com.sap.aii.mappingtool.tf7.IllegalInstanceException: Cannot create target element /ns0:MT_AutoPurchaseOrder_IB/PurchaseOrderNumber. Values missi~</text>

            </s:SystemError>

         </detail>

      </SOAP:Fault>

   </SOAP:Body>

</SOAP:Envelope>

Does anyone knows where is the problem?

Thanks In advence,

Amar