Content placeholder for meta description

Hi,
I have put a content place holder in my master page (in the head), and also in a page, with the same id (to show different descriptions)
I have a problem.
First;
<head id="Head1" runat="server">
<asp:Content ID="Content99" ContentPlaceHolderID="ContentPlaceHolder99" Runat="Server">

</asp:Content>
 
gives the error; 'unrecognized tag prefix 'asp'..
 
Thanks all 
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