How to retrive value from soap response jmeter

I have this soap response and i need to get the value of the tag platformMessage i writed a xpath query but it doesnt work, checking the log file in jmeter in tells me this "Prefix must resolve to a namespace: ns2", heres the xpath query

/S:Envelope[@xmlns:S="http://ift.tt/sVJIaE"]/S:Body/ns2:activateProductResponse[@xmlns:ns2="http://ift.tt/2a6DmQk"]/return/platformMessage

heres the xml

<S:Envelope xmlns:S="http://ift.tt/sVJIaE">
   <S:Body>
      <ns2:activateProductResponse xmlns:ns2="http://ift.tt/2a6DmQk">
         <return>
            <platformCode>1</platformCode>
            <platformMessage>Fail operation.El set-top box ya existe. Code Response:|22098: 22098: 1</platformMessage>
            <responseCode>13</responseCode>
            <responseMessage>Error executing action in platforn</responseMessage>
            <UUID>3cb49b29-513e-11e6-b5db-005056807f0c</UUID>
            <platformName>INTRAWAY</platformName>
         </return>
      </ns2:activateProductResponse>
   </S:Body>
</S:Envelope>