Rounded Corner extender don't work with me

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C#

ASP.NET

Ajax

dears,

i create a website , it is contain a master page , the main issue that i want to make the content place holder's corners be rounded.........

in the beginning of the code i insert ToolkitScriptManager.

<form id="form1"  runat="server" >
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</asp:ToolkitScriptManager>

Also i inserted a panel with content place holder .

<asp:Panel ID="Panel1" runat="server" BackColor="white">
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
</asp:Panel>

and finally i add the rounded corners extender

<asp:RoundedCornersExtender ID="RoundedCornersExtender1" runat="server" BorderColor="Black" Radius="10" TargetControlID="Panel1">
</asp:RoundedCornersExtender>
</form>

when i run my website , the panel appeared with it's color [white] in each page but not rounded .........WHY ??

NOTE : when i press on TargetControlID in the properties menu , panel 1 control not appeared , i type it in HTML code .