Object reference not set to an instance of an object.

Делаю обучающую прогу из книжки:

using System;
using System.Data;
using System.Configuration;
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;

public partial class _Default : System.Web.UI.Page
{
Class1 flashcard;
protected void Page_Load(object sender, EventArgs e)
{

flashcard = (Class1)Session["flasfcard"];
RefreshDisplay();
}

protected void TextBox1_TextChanged(object sender, EventArgs e)
{
if (TextBox1.Text == flashcard.answer().ToString())
{
Label1.Text = "Correct!";
flashcard.shuffle();
RefreshDisplay();
TextBox1.Text = "";
}
else
{
Label1.Text = "try again";


}
}
private void RefreshDisplay()
{
Label2.Text = flashcard.fn.ToString();
Label3.Text = flashcard.operation+flashcard.sn.ToString();
}
}

При запуске появляется сообщение об ошибке:
Server Error in '/WebSite1' Application.
Object reference not set to an instance of an object.