Prevent guests to see admin ASP.NET

hi all
i want to prevent other people except user and admin see panel pages
i tried to do this with session but i dont know why its not working
i have a table in my database called members with MemberId,MemberUser,MemberPass,MemberType
i want to check if membertype is "admin" redirect to adminpanel.aspx
if its user redirect to userpanel.aspx
else redirect to 404.aspx
and i have a login page named login.aspx
i used mdbl model linq to sql to do this
this is the code of the login button

    protected void Button3_Click(object sender, EventArgs e)
    {
        EshopDataClassesDataContext db = new EshopDataClassesDataContext();
        Member UserOne = db.Members.Where(t => t.MemberUser == txtUser0.Text && t.MemberPass == txtPass0.Text).FirstOrDefault();
        if (UserOne != null)
        {
            Session["MemberId"] = UserOne.MemberId.ToString();
            Session["MemberType"] = UserOne.MemberType;
            Session["MemberUser"] = UserOne.MemberUser;
            if (UserOne.MemberType == "Admin")
            {
                Response.Redirect("~/Admin/AdminPanel.aspx");
            }
            else if (UserOne.MemberType == "User") { Response.Redirect("~/User/UserPanel.aspx"); }
            else
            {
                Label1.Visible = true;
            }
        }
    }

and this is the code of page load of admin panel page


but it has eror like this

    Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error: 


Line 10:     protected void Page_Load(object sender, EventArgs e)
Line 11:     {
Line 12:         if (Session["MemberType"].ToString() == "Admin")
Line 13:         {
Line 14:             Response.Redirect("~/Admin/AdminPanel.aspx");

Source File: g:\ASP.net Capture\ASp\session 10\eshop\Admin\AdminPanel.aspx.cs    Line: 12 

what can i do ?
and very sry for bad english :(

protected void Page_Load(object sender, EventArgs e)
{
    if (Session["MemberType"].ToString() == "Admin")
    {
        Response.Redirect("~/Admin/AdminPanel.aspx");
    }
    else if (Session["MemberType"].ToString() == "User")
    {
        Response.Redirect("~/User/UserPanel.aspx");
    }
    else
    {
        Response.Redirect("~/404.asp");
    }
}