Unexpected case behaviour in Ruby

I got a wrong value in a very simple piece (as I thought) of code:

org = 4
case org
when org <= 1
  val = 'L'
when 2..3
  val = 'M'
when org >= 4
  val = 'H'
end

puts val
=> nil

Please, don't be angry, I expect that I missed something very obvious, but I really can't figure it out. Thanks.