Generate routes for nested resources with nil object

When we have nested routes like this:
map.resources :families, :has_many => :people

Inside controllers and views:
family_people_path(1) => /families/1/people
but:
family_people_path(nil) => /families//people

Is it not supposed to raise a RoutingError ?

In console it does:

include ActionController::UrlWriter
family_people_path(nil)
ActionController::RoutingError: family_people_url failed to generate from {:action=>"index", :family_id=>nil, :controller=>"people"}, expected: {:action=>"index", :controller=>"people"}, diff: {:family_id=>nil}