Undertow suppresses JAX-RS error message

I'm using a snapshot of WildFly 8.0.0.Alpha2 containing Undertow 1.0.0.Alpha17.

My RESTful Web Service in a war is using an ExceptionMapper (see below) which converts an exception into a 404 error code and also an error message. Now I try to submit a GET request resulting in an exception to be mapped.
I get the 404 error code, but NOT the error message. Instead I'm getting the contents of the error page which is declared in web.xml for a 404 error. In EAP 6.1.Beta I'm getting the error message produced by the exception mapper.

The exception mapper class:
@Provider
@ApplicationScoped
public class NotFoundExceptionMapper implements ExceptionMapper<NotFoundException> {
@Override
public Response toResponse(NotFoundException e)

{ final String msg = e.getMessage(); final Response response = Response.status(NOT_FOUND) .type(TEXT_PLAIN) .entity(msg) .build(); return response; }

}