Error page doesn't work

Hi,

I set the custom error page for my website but when I try to test the URL with .aspx it seems doesn't work. For example,if I trywww.exmaple.com/testempty, it return the custom error page. However if I try ww.example.com/testempty.aspx, it return 404 with unfriendly page. Why? Any suggestion will be appreciate.