ResolveUrl does not work with telerik

[Fernando]

Hi 
i have a problem with this code in a master page.  
<link href="<%=ResolveUrl("~/App_Themes/SkinSistema.css")%>" rel="stylesheet" type="text/css" />

before telerik, this work perfectly, but i have incorporate telerik RadScriptManager and then does not work.
this is my error:
"La colección de controles no puede modificarse porque el control contiene bloques de código (por ej. <% ... %>)."

i need help.
thk

[Fernando]

i have resolved this problem
removed the tag runat="server" of head.
i see this.
 http://www.telerik.com/community/forums/aspnet/editor/editor-ie-vs-firefox.aspx