Accessing session in integration test

I'm a Rails beginner working through Michael Hartl's Rails Tutorial and am receiving an error I have no idea how to fix. For reference, this is for implementing the changes in listing 9.24 (http://ift.tt/2sHvEFW).

I skipped chapter 9 (since it is supposedly optional) but in Chapter 10 it asks to include the changes made in listing 9.24.

This is the error I am recieving when I run rails test

Error:
UsersEditTest#test_unsuccessful_edit:
NoMethodError: undefined method `session' for nil:NilClass
    test/test_helper.rb:18:in `log_in_as'
    test/integration/users_edit_test.rb:14:in `block in <class:UsersEditTest>'


bin/rails test test/integration/users_edit_test.rb:12

E

Error:
UsersEditTest#test_successful_edit:
NoMethodError: undefined method `session' for nil:NilClass
    test/test_helper.rb:18:in `log_in_as'
    test/integration/users_edit_test.rb:28:in `block in <class:UsersEditTest>'

The tests (in test/integration/users_edit_test.rb) that are failing are:

test "successful edit" do
    log_in_as(@user)
    get edit_user_path(@user)
... end
test "unsuccessful edit" do
     log_in_as(@user)
    get edit_user_path(@user)
... end

and here is the integration/test_helper method that is being called

# Log in as a particular user.
  def log_in_as(user)
    session[:user_id] = user.id
  end

What is especially confusing is that there is another method in the test helper that also uses sessions, and is called in user_login_test which works fine.

Any help would be greatly appreciated!!