Use Fincontrol on an aspx form using c#

I am trying to use FindControl on aspx form using c#.  My code is as follows and FinControl returns null.  Thanks

<%@ Page Language="C#" MasterPageFile="~/Site1.Master" AutoEventWireup="true" Theme="Default" CodeBehind="Default.aspx.cs" Inherits="Application.Web.Default" Title="Information Database" %>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<table width="100%">
        <tr>
            <td>
               <div  ><asp:LinkButton Text="Clear" ID="ClearButton1" CssClass="clearButton" runat="server" OnClick="ClearButton_Onclick" CommandArgument="OfficeList"></asp:LinkButton></div>
            </td>

        protected void ClearButton_Onclick(object sender, EventArgs e)
        {//CommandArgument contains name of control to be found.
            LinkButton lb = sender as LinkButton;
             ListBox List = (ListBox)FindControl(lb.CommandArgument);
            List.SelectedIndex = -1;