I try to monitor a site that in case of failure responds with "forbidden"

In my monitor I have "Accepted HTTP status codes" set to 200.

Currently I expect the monitor to show me a failure, as the server answers with a  403 which is not 200...

But instead I get no monitoring at all and the message "configuration error: this url is forbidden".

How do I configure a monitor for such a forbidden message?

Thanks, Tim