Rails3 fields_for on text Hash: not working as expected

I have a rails3 form that allows the user to edit a list of answers, as part of an assessment.
I use a fields_for loop to generate each text input:

app/models/assessment.rb :

class Assessment < ActiveRecord::Base
  serialize :answers, Hash   # answers is a t.text field used to store all answers.
end

app/view/assessments/new.html.erb :

<p>Initialized answers: <%= @assessment.answers %></p>

<% item_counter = 0 %>
<% form.fields_for :answers do |answer_fields| %>
    <% item_id = "item" + item_counter.to_s %>
    <% item_counter = item_counter + 1 %>
    <div class="field">
      <%= answer_fields.label "the appropriate question, omitted for brevity" %>
      <br/>
      <% @assessment.answers[item_id] = "" %>
      <%= answer_fields.text_field item_id, :value => @assessment.answers[item_id] %>
    </div>
<% end %>

PROBLEM: The fields_for loop does zero iteration, no field gets printed.
( despite "Initialized answers:" showing correctly: {"a"=>143, "b"=>42} )