MASTER PAGE ERROR

Question

Hi,

I am getting this error when create master page

Error Rendering Control - ContentPlaceHolder1
An unhandled exception has occured
This control can only be used in a MasterPage.


The above shows in the masterpage design page

In the master page i have registered the below mentioned:
<%@ Register Assembly="System.Web.Silverlight" Namespace="System.Web.UI.SilverlightControls" TagPrefix="asp" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

I have a scriptmanager in the masterpage... What could be the problem... plzz help