What's the default IV for Cipher?

gistfile1.rb

require 'openssl'

data = "lesecret" * 10
cipher = OpenSSL::Cipher::AES256.new("CBC")
key = OpenSSL::Random.random_bytes(cipher.key_len)
cipher.encrypt
cipher.key = key

enc = cipher.update(data) + cipher.final

dec = OpenSSL::Cipher::AES256.new("CBC")
dec.decrypt
dec.key = key
dec.iv = "\0" * cipher.iv_len #this is the default IV, implied by OpenSSL itself

text = dec.update(enc) + dec.final

puts text == data  # => true