ruby-on-rails - Rails route helper not calling to_param

Question

I have a named route:

resources :groups do ... end

Groups have an overriden to_param method using the StringEx gem:

acts_as_url :name, sync_url: true

def to_param
  url
end

And most of the time, I can call it by passing a group in:

group_path(g)

But sometimes (and I haven't been able to find a clear link on the circumstances) it gives me this:

ActionController::RoutingError (No route matches {:action=>"show", :controller=>"groups", :id=>#<Group id: 121, name: "Example", kind: "ex" ... 

Really looks like it's not calling to_param, and is just calling to_s or inspect on the group.

Any idea what's going on? This is awfully frustrating.