redirecting 401.2 to custom page

I am trying to redirect my unauthorized users to a custom page, but it appears as though the redirect is getting stuck in an infinite loop! :(

I have the following in my Global.asax.vb page:
If (Response.StatusCode = 401 AndAlso Request.IsAuthenticated = True) Then
Response.ClearContent()
Response.Redirect("custom401.htm")
End If
any ideas WHY this would be happening? ...