The message received from the server could not be parsed.

I have used asp.net wizard inside update panel. Well in this, i have used combo box in second wizard and i have assigned integer character to combo. This gives an error while i press next button. But this works in 1st wizard page fine if i load it for the first time.
Please help to solve this problem. For this code is as below:
 
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="WizardTest.aspx.cs" Inherits="WizardTest" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
  &n ...