Create a SOAP-RPC message with xml as input

We have a flow service which takes xml as input (xmlRequest). The input xmlRequest is a string. In the flow service I use xmlStringToNode. When I send the SOAP request with xml as input, I am getting an error “Parameters xmldata, $filestream and $filedata are all missing” . To make it work, I had to encode the xml, by converting the < to < and > to >.
Is there any other way to make it work?