capistrano - NameError: uninitialized constant Net::SSH::KnownHosts::SUPPORTED_TYPE

I'm trying to deploy my Rails (3.1.3) application to the preprod env. I use capistrano (2.12.0) and rvm-capistrano (1.2.2). When I call `bundle exec cap ssh` it works fine. But when I call `bundle exec cap deploy` I get the following trace: ``` $ cap deploy triggering start callbacks for `deploy' * 18:42:19 == Currently executing `multistage:ensure' *** Defaulting to `preprod' * 18:42:19 == Currently executing `preprod' * 18:42:19 == Currently executing `deploy' * 18:42:19 == Currently executing `deploy:update' ** transaction: start * 18:42:19 == Currently executing `deploy:update_code' * 18:42:19 == Currently executing `deploy:set_previous_revision' * executing "cd /rails_apps/com.example.preprod/current; git rev-parse --short HEAD" servers: ["preprod.example.com"] connection failed for: preprod.example.com (NameError: uninitialized constant Net::SSH::KnownHosts::SUPPORTED_TYPE) ``` Of course `example.com` is a placeholder, it doesn't come from a mistake in the capistrano config. Any idea of what could cause that ? I'm using RVM with Ruby 1.9.3-p194. Thanks !