Undertow doesn't show the correct error page (401)

I'm using the WildFly snapshot from last Thursday with Undertow 1.0.1.
When I try to invoke a protected web page, I'm getting an almost blank web page with just the contents "401 - Unauthorized".
However, in WEB-INF/web.xml I'm having this declaration:

<error-page>
    <error-code>401</error-code>
    <location>/rf/error/accessDenied.jsf</location>
</error-page>