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

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:inrequire’
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:incall’
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:ineach’

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.