Change the SOAP header from web service call

Hi all,

I need to modify the XML header generated from a web service call. I have the following code:

data: ws_header    type ref to if_wsprotocol_ws_header.

data: lv_string    type string,
        lv_xstring  type xstring.

     concatenate
     '<soap-env:Envelope xmlns:soap env="http://schemas.xmlsoap.org/soap/envelope/"><soap-env:Header>'
     '</soap-env:Header>'
     '<soap-env:Body>' into lv_string.

* convert to xstring
       lv_xstring = cl_proxy_service=>cstring2xstring( lv_string ).
       if not lv_string is initial.

         call function 'SDIXML_XML_TO_DOM'
           exporting
             xml           = lv_xstring
           importing
             document      = xml_document
           exceptions
             invalid_input = 1
             others        = 2.

But when the function SDIXML_XML_TO_DOM is called, it always returns an invalid input error. Any Ideas?

Any help is much appreciated.