Ajax and masterpage issues

[egsdar]

Hello, I have a new issue between a new web form and Ajax Control Toolkit,
specifically a Tab Control, the thing is that I masterpage that i like to use
and this is the code:

<%@ Page Language="VB" MasterPageFile="~/MasterPage.master" Title="Untitled
Page" AutoEventWireup="true" Inherits="_Default" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit"
TagPrefix="cc1" %>

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder2"
Runat="Server">
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>

<br />
<cc1:TabContainer ID="TabContainer1" runat="server"
ActiveTabIndex="0" Style="position: static"><cc1:TabPanel runat="server"
HeaderText="TabPanel1" ID="TabPanel1"></cc1:TabPanel>
</cc1:TabContainer>
</asp:Content>


And is displaying this error:

Parser Error
Description: An error occurred during the parsing of a resource required to
service this request. Please review the following specific parse error
details and modify your source file appropriately.

Parser Error Message: The base class includes the field 'ScriptManager1',
but its type (AjaxControlToolkit.ToolkitScriptManager) is not compatible with
the type of control (System.Web.UI.ScriptManager).

Source Error:


Line 4:
Line 5: <asp:Content ID="Content1"
ContentPlaceHolderID="ContentPlaceHolder2" Runat="Server">
Line 6: <asp:ScriptManager ID="ScriptManager1" runat="server">
Line 7: </asp:ScriptManager>
Line 8:

How can I solve this?
Thanks.