I’m having trouble getting Supervisor.start_child to work and I get the following error when running these commands (the supervisor is started automatically. Any help is appreciated. I’m likely missing something simple.

import Supervisor.Spec
sup_pid = sup_pid = Process.whereis(App.SessionSupervisor)
Supervisor.start_child(sup_pid,["a"])

{:error,
   {:EXIT,
    {:undef,
      [{App.Session, :start_link, [["a"]], []},
      {:supervisor, :do_start_child_i, 3, [file: 'supervisor.erl', line: 374]},
      {:supervisor, :handle_call, 3, [file: 'supervisor.erl', line: 399]},
      {:gen_server, :try_handle_call, 4, [file: 'gen_server.erl', line: 629]},
      {:gen_server, :handle_msg, 5, [file: 'gen_server.erl', line: 661]},
      {:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line: 240]}]}}}

session_supervisor.ex

defmodule App.SessionSupervisor do
  use Supervisor

  def start_link do
    Supervisor.start_link(__MODULE__, :ok, name: App.SessionSupervisor)
  end

  def init(:ok) do
    children = [worker(App.Session, [], restart: :transient)]
    worker(App.Session, [], restart: :transient)

    supervise(children, strategy: :simple_one_for_one)
  end
end

session.ex

defmodule App.Session do

  use GenServer

  @doc """
  Starts a new session.
  """
  def start_link(uuid, opts \\ []) do
    GenServer.start_link(__MODULE__, %{uuid: uuid})
  end

… rest of the code.