asp.net ajax "This control cannot be displayed because TagPrefix is not registered to web Form "

<%@ Page Language="C#" MasterPageFile="~/Inv_MainMaster.master" AutoEventWireup="true" CodeFile="Inv_Issue.aspx.cs" Inherits="Default4" Title="Untitled Page" %>

<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="asp" %>

<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">

<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</asp:ToolkitScriptManager>
<table class="style2">
<tr>
<td>
<asp:Label ID="Label3" runat="server" Text="Prefix"></asp:Label>
</td>
<td>
<asp:DropDownList ID="txtPrefix" runat="server">
</asp:DropDownList>
</td>
<td>
<asp:Label ID="Label4" runat="server" Text="Date"></asp:Label>
</td>
<td>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
</td>
<td>
&nbsp;</td>
<td>
&nbsp;</td>
</tr>
<tr>
<td>
<asp:Label ID="Label5" runat="server" Text="Warehouse"></asp:Label>
</td>
<td>
<asp:ComboBox ID="ComboBox1" runat="server" AutoCompleteMode="Suggest"
DataSourceID="SqlDataSource1" DataTextField="xware" DataValueField="xware"
DropDownStyle="DropDownList" MaxLength="0" style="display: inline;">
</asp:ComboBox>
<asp:SqlDataSource ID="SqlDataSource1" runat="server"
ConnectionString="<%$ ConnectionStrings:ItechSoftERP %>"
SelectCommand="SELECT [xware] FROM [gstr]"></asp:SqlDataSource>
</td>
<td>
<asp:Label ID="Label6" runat="server" Text="Notes"></asp:Label>
</td>
<td>
<ajaxToolkit:ComboBox ID="ComboBox1" runat="server"
AutoPostBack="False"
DropDownStyle="DropDownList"
AutoCompleteMode="SuggestAppend"
CaseSensitive="False"
CssClass=""
ItemInsertLocation="Append">&nbsp;</td>
<td>
&nbsp;</td>
<td>
&nbsp;</td>
</tr>
<tr>
<td>
&nbsp;</td>
<td>
&nbsp;</td>
<td>
&nbsp;</td>
<td>
&nbsp;</td>
<td>
&nbsp;</td>
<td>
&nbsp;</td>
</tr>
<tr>
<td>
&nbsp;</td>
<td>
&nbsp;</td>
<td>
&nbsp;</td>
<td>
&nbsp;</td>
<td>
&nbsp;</td>
<td>
&nbsp;</td>
</tr>
</table>

</asp:Content>

i want to used 

http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/ComboBox/ComboBox.aspx

combobox auto suggestion

when i used this code

<ajaxToolkit:ComboBox ID="ComboBox1" runat="server" 
     AutoPostBack="False" 
     DropDownStyle="DropDownList" 
     AutoCompleteMode="SuggestAppend" 
     CaseSensitive="False" 
     CssClass="" 
     ItemInsertLocation="Append">

its making error This control cannot be displayed because TagPrefix is not registered to web Form

any help