Broken YAML.dump with YAML::Object (Missing String#each)

I feel like I've discovered a bug in YAML.dump when used with
yaml-tagged objects (YAML::Object). It seems that the YAML libs are
still expecting to use String#each, but that's gone. To repro:

    ruby -ryaml -e 'anon_obj_yaml = "---
!ruby/object:No::Such::Class"; anon_obj = YAML.load( anon_obj_yaml );
YAML.dump( anon_obj )'

Is this a bug?

jon