[xsl] Use document() with asp

I have an asp page able to inizialize the sessionId value 

Sessione.asp

<%
response.write "<?xml version='1.0' encoding='utf-8'?>"
response.write "<session_id>"&session.sessionId&"</session_id>"
%>
-----------


In my xsl 1.0 I call the value inside the template:

<xsl:value-of select="document('sessione.asp')//session_id" />

But it goes on error:

Nome con carattere iniziale non valido. 

Name with character not valid...

It'is necessary open only document with .xml extension ?
Thanks in advance.