trouble with blocking folder access...

In my web application, I have an Admin folder that I want to be accessible to only those users who have the Admin role.  So, in my main web.config file, I wrote:
 
<location path="~/Admin">
        <system.web>
            <authorization>
                <deny users="*" />
                <allow roles="Admin"/>
            </authorization>
        </system.web>
    </location>
 
However, it is not blocking the access.  Any help with this would be great...

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