Using Checkbox in gridview.. show the error when retriveing data from gridview....

Here is code....
Its show the error
Input string was not in a correct format.
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
      <div Align ="center" >
        <asp:GridView ID="gvFiles" runat="server" AutoGenerateColumns="False" AllowPaging="True" PageSize="18">
            <Columns>
               <asp:TemplateField>
    <ItemTemplate>
    <asp:CheckBox ID="chkSelect" runat="server" />
    </ItemTemplate>
    </asp:T ...