The following will fail if registration is required:

serializer.serialize(List(1,2,3))

This is because the collections list is missing :: and only has the empty list:

 // List is a sealed class, so there are only two subclasses:
      .forTraversableSubclass(List.empty[Any])

The fix is just to add list, but it needs to be added to the end or all of the registration numbers will change and it will break clients.