application_error wont' fire

[Phil]

I'm trying to add custom error handling for a 401 error. I've added this to
my web.config:
<location path="admin">
<system.web>
<authorization>
<allow roles="[domain]\[group]" />
<deny users="*" />
</authorization>
</system.web>
</location>

And this to my global.asax.cs Application_Error function:
Exception ex = Server.GetLastError().GetBaseException();

if (ex.GetType() == Type.GetType("System.UnauthorizedAccessException") )
{
Server.ClearError();
Response.Redirect(siteURL + "/401.aspx");
}

The login prompts as it should when I try to access the admin directory.
But, when I fail the login it gives me the generic 401 page. When I try to
debug it appears that the Application_Error function is never called.

Thanks for any help.

-Phil