invalid postback or callback error

Question

i found Invalid callback or postback propety error when we are submitting the button, actually we used bootstrap template on aspx page. Please tell me how to remove thios error.

?<%@ page language="C#" masterpagefile="~/MasterPage.master" EnableEventValidation="false" ValidateRequest="false" CodeFile="~/Contact_us.aspx.cs" inherits="Contact_us" %>  <asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server"> <!--start contact--> <div class="contactmap"> 	     <!--start info contact-->     <div class="infocontact infocontactdark blue">          	<div class="contentinfocontact">         	<p class="titleinfocontact">SKYLINK HOLIDAY</p>             <ul>             	<li><p><img alt="" src="img/contact/iconinfocontactaddress.png">B-113 Patel Garden Dwarka metro   </p></li>                 <li><p><img alt="" src="img/contact/iconinfocontactphone.png">Telephone: [phone]</p></li>                 <li><p><img alt="" src="img/contact/iconinfocontactmail.png">Mail: [email protected]</p></li>             </ul>         </div>                  <div class="triangleinfocontact">         	<span></span>         </div>              	<!--start marker-->         <div class="markercontactmap">             <div class="circlemarker"><div class="innercirclemarker"></div></div>             <div class="trianglemarker"></div>         </div>         <!--end marker-->          </div>     <!--end info contact-->      	<!--google maps-->     <div id="map-canvas" >          <img src="img/tours/contactus.jpg" style="width:100%; height:500PX"/> </div>     <!--google maps-->     	 </div> <!--end contactmap-->     <!--start page--> <section id="internalpage">  	<!--start container-->     <div class="container clearfix">     	         <!--start form-->         <form class="contactform">                      <div class="grid_4 red">                 <ul>                     <li class="filterinputicon"><asp:Label ID="name_lbl" runat="server" Text="Name" Font-Bold="True" ForeColor="Black" Font-Size="Large"></asp:Label></li>                     <li><asp:TextBox ID="name_txt" runat="server"></asp:TextBox></li>                 </ul>             </div>                                                    <div class="grid_4 orange">                 <ul>                     <li class="filterinputicon"><asp:Label ID="Label1" runat="server" Text="Email" Font-Bold="True" ForeColor="Black" Font-Size="Large"></asp:Label></li>                     <li><asp:TextBox ID="email_txt" runat="server"></asp:TextBox></li>                 </ul>             </div>                                                    <div class="grid_4 blue">                 <ul>                     <li class="filterinputicon"><asp:Label ID="Label2" runat="server" Text="Mobile" Font-Bold="True" ForeColor="Black" Font-Size="Large"></asp:Label></li>                     <li><asp:TextBox ID="mbl_txt" runat="server"></asp:TextBox></li>                 </ul>             </div>                                                    <div class="grid_12">                 <asp:Label ID="DisplayMessage" runat="server" Visible="false" />              <asp:TextBox ID="msg_txt" runat="server" TextMode="MultiLine" ></asp:TextBox>             </div>                          <div class="grid_12">              <asp:Button ID="send_btn" runat="server" Text="Send" OnClick="send_btn_Click"/>             </div>                  </form>         <!--end form-->          </div>     <!--end container-->       </section> <!--end internal page--> </asp:Content>