(RuntimeError) environment variable WOLFRAM_APP_ID is missing

Questions & Help Questions
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I’m reading the ebook Programming Phoenix 1.4 and in the page 268 when I try to execute the iex it complains for this:

** (RuntimeError) environment variable WOLFRAM_APP_ID is missing.

    (stdlib) erl_eval.erl:680: :erl_eval.do_apply/6
    (stdlib) erl_eval.erl:449: :erl_eval.expr/5
    (stdlib) erl_eval.erl:126: :erl_eval.exprs/5
    (elixir) lib/code.ex:240: Code.eval_string/3
    (mix) lib/mix/config.ex:158: anonymous fn/2 in Mix.Config.__import__!/2
    (elixir) lib/enum.ex:1948: Enum."-reduce/3-lists^foldl/2-0-"/3
    (mix) lib/mix/config.ex:157: Mix.Config.__import__!/2

So I add the API key in both config/dev.exs and config/prod.secret.exs files:

wolfram_app_id = 
  System.get_env("XXXXXX-XXXXXXXXXX") || 
  raise """
  environment variable WOLFRAM_APP_ID is missing.
  """
config :info_sys, :wolfram, app_id: wolfram_app_id

I created the key XXXXXX-XXXXXXXXXX in the http://developer.wolframalpha.com/portal/myapps/index.html
with the Application name: rumbl
and then generated the AppID: XXXXXX-XXXXXXXXXX
What I’m doing incorrect?
the value of the WOLFRAM_APP_ID I put just on the config/prod.secret.exs file?
like WOLFRAM_APP_ID= "XXXXXX-XXXXXXXXXX"
and then keep on the config/dev.exs file the code: System.get_env("WOLFRAM_APP_ID") || ?