Get request XML with process instructions in WCF Rest service

The HTTP POST request XML:

<?xml version = '1.0' encoding = 'UTF-8'?>
<?Label FSDH|RESULT|1|SUCCESS?>
<RESULT xmlns="result.fidelio.1.0" success="SUCCESS">
 <resortId>FSDH</resortId>
</RESULT>

I was trying to get the request xml by using OperationContext.

string xml = OperationContext.Current.RequestContext.RequestMessage.ToString();

And I get the following error:

Error reading body: System.Xml.XmlException: Processing instructions (other than the XML declaration) and DTDs are not supported. Line 2, position 2

Anyone could help to get the request XML with process instructions? Thanks.