Inheriting Renderer breaks all named renderer calls

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In my template I have :

{% block navbar %}
{{nav.top.render(renderer='simple')}}
{% endblock %}

And a very basic navbar:

def create_navbar():
    return Navbar(View('Index', 'index.index'))

I inherit and register a renderer:

class FormRenderer(Renderer):
    pass

register_renderer(app, 'form', FormRenderer)

Then, the call to the render(renderer='simple') does not create the navbar, whereas the call to render() will create it.

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Maybe just a documentation bug and something I do not clearly understand ?
Havig a look with the debugger it seems that BootstrapRenderer is the default, not SimpleRenderer...