[Solved] document.getElementById returns NULL when using Master Page in ASP.Net

Refer here:

http://www.aspforums.net/Threads/136592/Copy-Text-from-one-TextBox-to-another-while-it-is-typed-using-JavaScript/

It is not working in my codes:

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
  <script type="text/javascript">
      function OnTextKeyUp(txt) {
          document.getElementById("TextBox2").value = txt.value;
      }
    </script>
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    
    <table cellpadding="5" cellspacing="5">
        <tr>
            <td>
                Email:
            </td>
            <td>
                <asp:TextBox ID="TextBox1" runat="server" onkeyup="OnTextKeyUp(this);"></asp:TextBox>
            </td>
        </tr>
        <tr>
            <td>
                Confirm Email:
            </td>
            <td>
                <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
            </td>
        </tr>
    </table>
</asp:Content>

Thanks~