Setting Session Variable from UpdatePanel

I am using ASP.NET 2.0 AJAX Extensions 1.0 with the version v1.0.20229 of the AJAX Control Toolkit (which to my knowledge is the latest for .NET 2.0/Visual Studio 2005).

My web page (aspx) has a DropDownList control on an UpdatePanel. In the handler for the DropDownList's SelectedIndexChanged event I attempt to set a session variable.

The first time the event is fired, I get a Sys.WebForms.PageRequestManagerParserErrorException: "The message received from the server could not be parsed". If I continue, subsequent SelectedIndexChanged's are handled successfully.

I have stumbled upon a solution whereby if I initialise the session variable in my Page_Load (so the event handler is just setting the value of a session variable that already exists as opposed to creating a new one) the problem goes away.

I'm happy to do this, but I'm curious as to exactly what the underlying cause is. Can anyone explain?

(My suspicion is that setting the session variable receives a response from the server which is then returned to the 'caller', but it's not the sort of response it knows how to deal with causing the exception?)

EDIT: I reproduced the problem in a seperate little project:

Default.aspx

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="SessionTest._Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"&gt;

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">    
        <asp:ScriptManager ID="ScriptManager1" runat="server" />

        <div>

            <asp:UpdatePanel id="upCategorySelector" runat="server">
                <ContentTemplate>

                    Category:
                    <asp:DropDownList ID="ddlCategory" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlCategories_SelectedIndexChanged">
                        <asp:ListItem>Item 1</asp:ListItem>
                        <asp:ListItem>Item 2</asp:ListItem>
                        <asp:ListItem>Item 3</asp:ListItem>
                    </asp:DropDownList>

                </ContentTemplate>
            </asp:UpdatePanel>

        </div>

    </form>
</body>
</html>

Default.aspx.cs

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

namespace SessionTest
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            // If I do this, the exception does not occur.
            if (Session["key"] == null)
                Session.Add("key", 0);
        }

        protected void ddlCategories_SelectedIndexChanged(object sender, EventArgs e)
        {
            // If Session["key"] has not been created, setting it from
            // the async call causes the excaption
            Session.Add("key", ((DropDownList)sender).SelectedValue);
        }
    }
}

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Setting Session Variable from UpdatePanel

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