.Net custom error default redirect not working with url encoding /%20/

Hi,  We have a .Net 4.0 web site running on IIS 7.5.  We have custom error turned on to mask any unknown error.  Like this:  <customErrors defaultRedirect="customerror.htm" mode="On" />  

When we tested the custom error out by going to an invalid URL, we got redirected to the customerror.htm page, which is the expected behavior.   But when we tested the URL in this format -> "https://<sitefqdn>/<subsite>/ /"  (It includes a space in between 2 slashes, when entered onto the browser's address bar, it is encoded to %20), IIS returned a .Net error instead of the custom error.

We tried masking it with various customErrors configurations, with URL rewrite/filtering, and with HTTP module, but none of these methods seem to work. 

Any recommendations?