Custom Errors Not Working for Empty Directories

I recently upgraded from IIS 5 to IIS 7.5 (Server 2008 R2) and I am having trouble getting the same custom error behavior.

• mysite.com/i_dont_exist/: Correctly displays my custom error page.
• mysite.com/i_dont_exist/index.aspx: Correctly displays my custom error page.
• mysite.com/i_dont_exist/default.aspx: Correctly displays my custom error page.
• mysite.com/i_dont_exist/index.htm: Correctly displays my custom error page.
• mysite.com/i_dont_exist/anything_else/: Correctly displays my custom error page.

In IIS 5, unless I actually put an index or default file in a directory, IIS would always display my custom error page. However, in IIS 7.5 if I create an empty directory, here are my results:

• mysite.com/empty_directory/: Completely Blank Page (200 OK Status)
  
• mysite.com/empty_directory/index.aspx: Correctly displays my custom error page.
• mysite.com/empty_directory/default.aspx: Correctly displays my custom error page.
• mysite.com/empty_directory/index.htm: Correctly displays my custom error page.
• mysite.com/empty_directory/anything_else/: Correctly displays my custom error page.

My entire web.config looks like this:
---------------------------------------------------
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.web>
        <trust level="Full" />
    </system.web>
   
    <system.webServer>
        <httpErrors errorMode="Custom" existingResponse="Replace">
            <remove statusCode="404" subStatusCode="-1" />
            <remove statusCode="403" subStatusCode="14" />
            <error statusCode="404" subStatusCode="-1" prefixLanguageFilePath="" path="/" responseMode="ExecuteURL" />
            <error statusCode="403" subStatusCode="14" prefixLanguageFilePath="" path="/" responseMode="ExecuteURL" />
        </httpErrors>
        <staticContent>
            <mimeMap fileExtension=".otf" mimeType="application/octet-stream" />
        </staticContent>
    </system.webServer>

</configuration>
---------------------------------------------------

As you can see, I am using the web site root as my custom error page, but it works the same way if I use something like path="/404.aspx".

Failed Request Tracing gives me the following on a request to an empty directory:
---------------------------------------------------
273. -NOTIFY_MODULE_START
ModuleName CustomErrorModule
Notification 536870912
fIsPostNotification false
Notification SEND_RESPONSE

274. -GENERAL_NOT_SEND_CUSTOM_ERROR
Reason 1
Reason SETSTATUS_SUCCESS
---------------------------------------------------

I have been searching for days on this so any help/hints/suggestions would be greatly appreciated.

fodder.