Error in log file when cancelling a PO using standard API

When trying to cancel a PO using standard po_document_control_pub WSDL I get the error message in log:"Unable to convert the XSD element P_API_VERSION whose SQL type is NUMBER and JDBC type is NUMERIC. Cause: java.lang.NumberFormatException"

Sample XML:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:po="http://xmlns.oracle.com/apps/po/soaprovider/plsql/po_document_control_pub/" xmlns:con="http://xmlns.oracle.com/apps/po/soaprovider/plsql/po_document_control_pub/control_document/">

   <soapenv:Header>

      <po:SOAHeader>

         <po:Responsibility></po:Responsibility>

         <po:RespApplication></po:RespApplication>

         <po:SecurityGroup></po:SecurityGroup>