Getting Plug.Conn.AlreadySentError but only in tests

I’m using Programming Phoenix by Chris McCord as a reference to build my personal blog. I’ve replicated the authentication implementation almost exactly, but now that I am working through the chapter about testing the authentication plug, I’m getting (Plug.Conn.AlreadySentError) the response was already sent. However, the site works fine when navigating through it regularly, just not in the tests. I’ve looked the problem up and ensured I don’t have plug :action anywhere and am calling halt() after I redirect when an action isn’t authorized. If anyone could help I’d really appreciate it Relevant code:

  1) test login puts the user into the session (PhoenixBlog.AuthTest)
     test/plugs/auth_test.exs:28
     ** (Plug.Conn.AlreadySentError) the response was already sent
     stacktrace:
       (plug) lib/plug/conn.ex:862: Plug.Conn.put_session/3
       (phoenix_blog) web/plugs/auth.ex:25: PhoenixBlog.Plugs.Auth.login/2
       test/plugs/auth_test.exs:31: (test)

  2) test authenticate_user halts when no current_user exists (PhoenixBlog.AuthTest)
     test/plugs/auth_test.exs:14
     ** (Plug.Conn.AlreadySentError) the response was already sent
     stacktrace:
       (plug) lib/plug/conn.ex:607: Plug.Conn.put_resp_header/3
       (phoenix) lib/phoenix/controller.ex:303: Phoenix.Controller.redirect/2
       (phoenix_blog) web/plugs/auth.ex:57: PhoenixBlog.Plugs.Auth.authenticate_user/2
       test/plugs/auth_test.exs:15: (test)

Notice that only the tests which hit the current_user doesn’t exist clause in authenticate_user fail.

defmodule PhoenixBlog.AuthTest do
  use PhoenixBlog.ConnCase
  alias PhoenixBlog.Plugs.Auth

  setup %{conn: conn} do
    conn =
      conn
      |> bypass_through(Rumbl.Router, :browser)
      |> get("/")

    {:ok, %{conn: conn}}
  end

  test "authenticate_user halts when no current_user exists", %{conn: conn} do
    conn = Auth.authenticate_user(conn, [])
    assert conn.halted
  end

  test "authenticate_user continues when the current_user exists", %{conn: conn} do
    conn =
      conn
      |> assign(:current_user, %PhoenixBlog.User{})
      |> Auth.authenticate_user([])

    refute conn.halted
  end

  test "login puts the user into the session", %{conn: conn} do
    login_conn =
      conn
      |> Auth.login(%PhoenixBlog.User{id: 123})
      |> send_resp(:ok, "")

    next_conn = get(login_conn, "/")
    assert get_session(next_conn, :user_id) == 123
  end
end

Auth plug:

defmodule PhoenixBlog.Plugs.Auth do
  import Plug.Conn
  import Comeonin.Bcrypt, only: [checkpw: 2, dummy_checkpw: 0]

  def init(opts) do
    Keyword.fetch!(opts, :repo)
  end

  def call(conn, repo) do
    user_id = get_session(conn, :user_id)

    cond do
      user = conn.assigns[:current_user] ->
        conn
      user = user_id && repo.get(PhoenixBlog.User, user_id) -> 
        assign(conn, :current_user, user)
      true ->
        assign(conn, :current_user, nil)
    end
  end

  def login(conn, user) do
    conn
    |> assign(:current_user, user)
    |> put_session(:user_id, user.id)
    |> configure_session(renew: true)
  end

  def logout(conn) do
    configure_session(conn, drop: true)
  end

  def login_by_username_and_pass(conn, username, given_pass, opts) do
    repo = Keyword.fetch!(opts, :repo)
    user = repo.get_by(PhoenixBlog.User, username: username)

    cond do
      user && checkpw(given_pass, user.password_hash) ->
        {:ok, login(conn, user)}
      user ->
        {:error, :unauthorized, conn}
      true ->
        dummy_checkpw()
        {:error, :not_found, conn}
    end
  end

  import Phoenix.Controller
  alias PhoenixBlog.Router.Helpers

  def authenticate_user(conn, _opts) do
    if conn.assigns.current_user do
      conn
    else
      conn
      |> put_flash(:error, "You must be logged in to access that page")
      |> redirect(to: Helpers.page_path(conn, :index))
      |> halt()
    end
  end
end