Rails 4: Nested Attributes Not Showing on Form

I am trying to run through a simple nested_attributes exercise, where a question has many answers (how like life is that?). I can't get the answers field to show up in either the 'new' or the 'edit' actions. As far as I can tell, I'm following the specs precisely. Clearly, this is not the case. Any pointers appreciated, thanks.

Models

# question.rb
has_many :answers
accepts_nested_attributes_for :answers

# answer.rb
belongs_to :question

View

# _form.html.erb
<%= form_for(@question) do |f| %>
  ...
  <div class="field">
    <%= f.label :content, 'Question' %><br>
    <%= f.text_field :content %>
  </div>

  <div class="field">
        <% f.fields_for :answers do |ff| %>
        <%= ff.label :content, 'Answer' %><br>
        <%= ff.text_field :content %>
        <% end %>
  </div>
  ...
<% end %>

Controller

# questions_controller.rb
def new
  @question = Question.new
  @question.answers.build
end

def edit
end

def create
  @question = Question.new(question_params)

  respond_to do |format|
    if @question.save
      format.html { redirect_to @question, notice: 'Question was successfully created.' }
      format.json { render :show, status: :created, location: @question }
    else
      format.html { render :new }
      format.json { render json: @question.errors, status: :unprocessable_entity }
    end
  end
end

private 

def question_params
  params.require(:question).permit(:content, answer_attributes: [:id, :content, :question_id] )
end