I've replaced Umbraco's default RenderMVCController using the last section here. I would like to return the same view regardless if Umbraco finds a template or not (this view returns a single page app):

public class DefaultRenderMvcController : RenderMvcController
{
    public override ActionResult Index(ContentModel model)
    {
        return View("~/Views/DefaultPage/Index.cshtml", model);
    }
}

Since my document types doesn't have templates (they are managed in the single page app), I get the default Umbraco 404 page.

How can I make the 404 request end up in my DefaultRenderMvcController? Right now Index() isn't called.