Anomaly in poison decoder

poison_anomaly.exs

defmodule Payment do
  @derive {Poison.Encoder, except: [:entity, :order_id]}

  defstruct [:id, :amount, :currency, :status, :method, :description,
  :refund_status, :amount_refunded, :email, :contact, :fee, :service_tax,
  :error_code, :error_description, :notes, :created_at]
end

defmodule Collection do
  @derive {Poison.Encoder, except: [:entity]}

  defstruct [:count, :items]
end

defmodule PoisonAnomaly do
  def run do
    str = "{\"entity\":\"collection\",\"count\":2,\"items\":[{\"id\":\"fake_id1\",\"entity\":\"payment\",\"amount\":5000,\"currency\":\"INR\",\"status\":\"failed\",\"order_id\":null,\"method\":\"netbanking\",\"amount_refunded\":0,\"refund_status\":null,\"captured\":false,\"description\":\"Purchase Description\",\"card_id\":null,\"bank\":\"HDFC\",\"wallet\":null,\"email\":\"test@test.com\",\"contact\":\"+919123456789\",\"notes\":[],\"fee\":0,\"service_tax\":0,\"error_code\":\"BAD_REQUEST_ERROR\",\"error_description\":\"Payment failed\",\"created_at\":1471720420},{\"id\":\"fake_id2\",\"entity\":\"payment\",\"amount\":5000,\"currency\":\"INR\",\"status\":\"captured\",\"order_id\":null,\"method\":\"netbanking\",\"amount_refunded\":4850,\"refund_status\":\"partial\",\"captured\":true,\"description\":\"Purchase Description\",\"card_id\":null,\"bank\":\"HDFC\",\"wallet\":null,\"email\":\"test@test.com\",\"contact\":\"+919123456789\",\"notes\":[],\"fee\":144,\"service_tax\":19,\"error_code\":null,\"error_description\":null,\"created_at\":1471720235}]}"

    Poison.decode!(str, keys: :atoms!, as: %Collection{items: [%Payment{}]})
  end
end

PoisonAnomaly.run