Lumen - Routing with Prefix and Optional Parameter

I'm looking at routing in Lumen and it doesn't appear to be working correctly and I can't work out if it's an issue or my understanding.

$router->get('{adaptor}[/{id}]',     ['uses' => 'MyController@readAction']);

This way works, but I'd prefer to use a prefix to group all my routes:

$router->group(['prefix' => '{adaptor}'], function () use ($router) {
        $router->get('[/{id}]',      ['uses' => 'MyController@readAction']);
       // CRUD routes to be added here
});

If I go to /acme I get MethodNotAllowed Exception, but if I add /acme/1 it works fine. Does a prefix NEED another route item for it to work?

via Chebli Mohamed