[Mac OS X Lion] Net::SCP and ssh-agent ?

[Une Bévue]

I'm starting to test Net::SCP and get the following error :

Text will be echoed in the clear. Please install the HighLine or Termios
libraries to suppress echoed text.
Enter passphrase for /Users/yt/.ssh/id_rsa:
NoMethodError: undefined method 'chomp' for nil:NilClass

In fact, usually i don't need any password or passphrase because i've
setup ssh-add at login.

How do I setup Net::SCP for that setting ?