Custom 404 Page Not served for extensionless URL

I am running into a situation,

My custom 404 page works just fine with URL that has extensions.

http://www.space.ca/ssss.aspx A custom page URL serves up as intended,

however, if you try the link without the .aspx extension http://www.space.ca/ssss

it goes to the IIS default page. Any idea on why?

This is my configuration , inside system.webserver

<httpErrors><remove statusCode="500" subStatusCode="-1" /><remove statusCode="404" subStatusCode="-1" /><error statusCode="404" prefixLanguageFilePath="" path="/errors/404.aspx" responseMode="ExecuteURL" /><error statusCode="500" prefixLanguageFilePath="" path="/errors/500.aspx" responseMode="ExecuteURL" /></httpErrors>