flash[:success] test is not passing unless I fake it out.

When I literally copy the code from the project file for the users_controller, I get this failed test:

"1) UsersController POST create with valid params sets the flash success message Failure/Error: expect(flash[:success]).to eq("Thanks for signing up!")

expected: "Thanks for signing up!"
got: nil

(compared using ==)"

Here's what I have in the users_controller under the create function:

def create
    @user = User.new(user_params)

    respond_to do |format|
      if @user.save
        session[:user_id] = @user.id
        flash[:success] = "Thanks for signing up!"
        format.html { redirect_to todo_lists_path, flash[:success]=>'Thanks for signing up!' }
        format.json { render :show, status: :created, location: @user }
      else
        format.html { render :new }
        format.json { render json: @user.errors, status: :unprocessable_entity }
      end
    end
   end

When I add this code above the format.html:

flash[:success] = "Thanks for signing up!"

then the test passes.

I'm pretty sure I'm passing the test, though, without getting the right code. Any ideas why the test doesn't pass with the project code as is?

Thanks