Cannot use a / in route parameter

According to the Symfony documentation it should be possible to use a "/" (forward slash) in a route path by following the instructions here https://symfony.com/doc/3.4/routing/slash_in_parameter.html

_hello:
    path:     /hello/{username}
    defaults: { _controller: AppBundle:Demo:hello }
    requirements:
        username: .+

I've tried this but a request to /hello/foo/bar didn't call my controller.