[chef] LoadError on 'chef_fs/config' for knife-ec-backup plugin

[Ken Miles < >]

Chefs,

I am attempting to test drive the knife-ec-backup plugin (https://github.com/jkeiser/knife-ec-backup) but am going nowhere fast.

I installed it by cloning and copying these to the plugins dir:

$ ls -1 .chef/plugins/knife/*rb
.chef/plugins/knife/ec_backup.rb
.chef/plugins/knife/ec_restore.rb

They appear to be installed:

$ knife -h | grep ' ec '
knife ec backup
knife ec restore

But trying a backup gives me:

knife ec backup backup_dir
/opt/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- chef_fs/config (LoadError)
    from /opt/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
    from /home/vagrant/.chef/plugins/knife/ec_backup.rb:13:in `block in <class:EcBackup>'
    from /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.8.0/lib/chef/knife.rb:215:in `call'
    from /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.8.0/lib/chef/knife.rb:215:in `block in load_deps'
    from /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.8.0/lib/chef/knife.rb:214:in `each'
...

But config is there:
wc /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.8.0/lib/chef/chef_fs/config.rb
 160  553 5762 /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.8.0/lib/chef/chef_fs/config.rb

I'm getting the same results on several hosts including a couple of CentOS6.4 boxes and a clean install to a Vagrant Ubuntu12.04 VM, all using 11.8.

What am I doing wrong?

Thanks,

-Ken M.