Session Object unavailabe

[Darshan Mehta]

I am creating a Webservice that saves the user information in the session
state instead of a cookie. In the web.config, I enabled session state and set
the timeout to "5" (for 5 minutes).
I have set the Authentication to windows. I have a client application call a
logon Method and pass the userID and Password, as parameters. In the logon
method, I check the userID and pasword against a SQL Server Database. Then I
attempt to add the username to the session object using the following 2 lines
og code:

Session.Add("UserID", null);
Session["UserID"] = uderID;

But the first line of code(Session.Add(...)) gives me a
NullReferenceException.
Here is what I have in the Web.Config:

<authentication mode="Windows" />
<authorization>
<allow users="*" />
</authorization>
<sessionState mode="InProc" cookieless="true" timeout="5"/>

Is there anything that I left or or am doing wrong?

Thanks in Advance,
Darshan