Form based authentication

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ASP.NET

<configuration>
<location path="admin">
    <system.web>
      <authentication mode="Forms">
        <forms cookieless="AutoDetect"
                name=".3D"
             slidingExpiration="true" >
          <credentials passwordFormat="Clear">
            <user name="Bill" password="secret_" />
            <user name="Ted" password="secret_" />
          </credentials>
        </forms>
      </authentication>
      <authorization>
        <deny users="?" />
      </authorization>
    </system.web>
 </location>
</configuration>

In my project i am using just one web.config.
This is the part of web.config which in root. and i am trying to protect admin subfolder. but it is giving the following error...

It is an error to use a section registered as allowDefinition='MachineToApplication' beyond application level. This error can be caused by a virtual directory not being configured as an application in IIS.