Friendly URL's and Application_Error

I love Friendly URL's feature that was introduced for ASP.Net WebForms. I find only one major issue so far: if you issue a request http://mysite/aaa.aspx custom error page that I have configured in web.config is shown. No surprises. But in case of http://mysite/aaa request I see IIS 404 error page .
Basically it means that Application_Error event is not firing. I wasn't able to find a work around so far. This is a pretty big issue and prevents normal error handling.  Any help is greatly appreciated.