Problema de Bitcoin Testnet con bitcoin-ruby

Tengo problemas con la gema bitcoin-ruby. Recibo el siguiente error cuando intento enviar dinero a otra dirección que tengo en la red de prueba:

/var/lib/gems/2.2.0/gems/bitcoin-ruby-0.0.7/lib/bitcoin/protocol/txout.rb:76:in pk_script=': undefined methodbytesize' for nil:NilClass (NoMethodError)

Estoy generando una clave privada con:

def new_address
 Bitcoin::generate_address
end

Y obtener detalles de la clave privada con:

def key_details(prikey, pubkey)
  #returns prikey, prikey_hash58, pubkey_hash58, pubkey_uncompressed, address as a hash
  my_key = Bitcoin::Key.new(prikey, pubkey)
  { prikey:prikey, 
    prikey_base58:my_key.to_base58, 
    pubkey_58:my_key.hash160, 
    pubkey: my_key.pub_uncompressed, 
    address:my_key.addr
  }
end

El código que tengo para enviarme dinero a mí mismo es el siguiente:

require 'bitcoin'
require_relative 'utilities.rb'
require 'open-uri'

Bitcoin.network = :testnet3

def build_transaction(prev_tx, prev_out_index, key, value, addr, message)
  include Bitcoin::Builder

  new_tx = build_tx do |t|
    t.input do |i|
      i.prev_out prev_tx
      i.prev_out_index prev_out_index
      i.signature_key key
    end
    t.output do |o|
      o.value value 
      o.script {|s| s.type :address; s.recipient addr }
    end
  end
end

def prev_tx(prev_hash, network)
  if network == "testnet3"
    prev_tx = Bitcoin::P::Tx.from_json(open("http://test.webbtc.com/tx/#{prev_hash}.json"))
  else
    prev_tx = Bitcoin::P::Tx.from_json(open("http://webbtc.com/tx/#{prev_hash}.json"))
  end
end

def key(publ_key, priv_key)
  key = Bitcoin::Key.new(priv_key, publ_key)
end

def bin_to_hex(s)
  s.unpack('H*').first
end

#transaction inputs
priv_key = "private_key"
publ_key = "public_key_long_format"
address = "address"
previous_tx = "previous_tx_hash"

# generate tx info off inputs
key = Bitcoin::Key.new(priv_key, publ_key)
prev_tx = prev_tx(previous_tx, "testnet3")
prev_out_index = 1
tx_value = prev_tx.outputs[prev_out_index].value

# build new tx
tx = build_transaction(prev_tx, prev_out_index, key, tx_value, address, "hello")

#
# puts tx.to_json
puts bin_to_hex(tx.to_payload)