asp:ScriptManager error ‘JScript runtime error: 'Sys' is undefined’

有的时候在用ajax拖拽<asp:ScriptManager   ID="ScriptManager1"   runat="server"> 到页面里面的时候，
</asp:ScriptManager> 但是运行时却出现了"Microsoft   JScript   runtime   error:   'Sys'   is   undefined"  


用的是ajax.net1.0正式版的话，要在web.config里面配置一下：

 <httpHandlers>
            <remove verb="*" path="*.asmx"/>
            <add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory"/>
            <add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="false"/>
        </httpHandlers>