Unclear how to setup custom error pages

Traefik Traefik v2
, ,
1

the documentation says it's easy to setup custom error pages:

apiVersion: traefik.containo.us/v1alpha1
kind: Middleware
metadata:
  name: test-errorpage
spec:
  errors:
    status:
      - 500-599
    query: /{status}.html
    service:
      name: whoami
      port: 80

but it's quite unclear how to setup a 404 error page.
if the url path doesn't match any provider path where and how should it be defined ?

best regards,
david