从系统环境加载端点配置

我在中进行了以下端点初始化 lib/flashcards_web/endpoint.ex :

  @doc """
  Callback invoked for dynamically configuring the endpoint.

  It receives the endpoint configuration and checks if
  configuration should be loaded from the system environment.
  """
  def init(_key, config) do
    if config[:load_from_system_env] do
      port = System.get_env("PORT") || raise "expected the PORT environment variable to be set"                                                      
      jwt_token_ttl_minutes =
        "USER_SESSION_MINUTES"
        |> System.get_env
        |> String.to_integer
      || raise "expected the USER_SESSION_MINUTES environment variable to be set"                                                                    

      config =
        config
        |> Keyword.put(:http, [:inet6, port: port])
        |> Keyword.put(:jwt_token_ttl_minutes, jwt_token_ttl_minutes)

      {:ok, config}
    else
      {:ok, config}
    end
  end

以及所需的 load_from_system_env: true 行在 config/dev.exs :

# For development, we disable any cache and enable
# debugging and code reloading.
#
# The watchers configuration can be used to run external
# watchers to your application. For example, we use it
# with brunch.io to recompile .js and .css sources.
config :flashcards, FlashcardsWeb.Endpoint,
  http: [port: 4000],
  debug_errors: true,
  code_reloader: true,
  check_origin: false,
  watchers: [node: ["node_modules/brunch/bin/brunch", "watch", "--stdin",
                    cd: Path.expand("../assets", __DIR__)]],
  load_from_system_env: true

但是在运行时

PORT=4000 USER_SESSION_MINUTES=1 iex -S mix phx.server

我得到:

iex(1)> Application.get_env(:flashcards, FlashcardsWeb.Endpoint)[:jwt_token_ttl_minutes]
nil

我是不是错过了什么?