Ajax AutoComplete is not responding after TextBox1.Focus() command

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ASP.NET

Ajax

I have TextBox and an AutoCompleteExtender targeted to this TextBox.
One button is also there. After clicking the button the AutoCompleteExtender is not working. When I remove one line of code (ie.txtTestCode.Focus();) it is working perfectly. But I can't remove this because after clicking the button again it should focus to TextBox. How can I resolve this?

<form id="form1" runat="server">
       <asp:ScriptManager ID="ScriptManager1" runat="server">
       </asp:ScriptManager>
       <div>
           <asp:TextBox ID="txtTestCode" runat="server"></asp:TextBox>
               <cc1:AutoCompleteExtender ID="AutoCompleteExtenderDemo" runat="server"
               TargetControlID="txtTestCode" ServiceMethod="GetCompletionList" EnableCaching="true"
               MinimumPrefixLength="1" CompletionInterval="10"
               CompletionSetCount="5">
               </cc1:AutoCompleteExtender>
           <asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
       </div>

   </form>

[System.Web.Services.WebMethod]
    public static string[] GetSuggestions(String prefixText, int count)
    {
        string[] movies = { "BIO", "CYT", "GTT", "HEM", "HTP", "MBI", "PHS", "RGL", "SEM", "SPL", "STL", "URN" };

        ArrayList filteredList = new ArrayList();

        foreach (string s in movies)
        {
            if (s.ToLower().StartsWith(prefixText.ToLower()))
                filteredList.Add(s);
        }
        return (string[])filteredList.ToArray(typeof(string));
    }

    [System.Web.Services.WebMethod]
    public static string[] GetCompletionList(String prefixText, int count)
    {
        string[] suggetions = GetSuggestions(prefixText, count);
        return suggetions;
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        txtTestCode.Text = "";
        txtTestCode.Focus();
    }