has_many + through + nested model

Hello Friends,
I am stucked with the nested model with through association
I have
class Office < AR::B
has_many :office_user_details
has_many :user_details, :through => :office_user_details
accepts_nested_attributes_for :user_details
end
class UserDetail < AR::B
has_many :office_user_details
has_many :offices, :through => :office_user_details
accepts_nested_attributes_for :offices
end
class OfficeUserDetail < AR::B
belongs_to :user_details
belongs_to :offices
end
FORM
<% form_for @user do |f| %>
<% f.fields_for :user_detail_attributes do |user_detail| %>

<%= user_detail.label :given_name %>


<%= user_detail.text_field :given_name %>


<% user_detail.fields_for :offices_attributes do |office| %>

<%= office.label :office_name %>


<%= office.text_field :office_name %>


<%end%>
<%end%>

<%= f.submit 'Sign Up' %>

<%end%> But getting the following error **undefined method `stringify_keys' for "Andy Inc":String (NoMethodError)**

Thanks
Abhishek

Does this work>

<% form_for @user do |f| %>

  <% f.fields_for :user_details do |user_detail| %>
      <p><%= user_detail.label :given_name %></p>
      <p><%= user_detail.text_field :given_name %></p>

      <% user_detail.fields_for :offices do |office| %>
        <p><%= office.label :office_name %></p>
        <p><%= office.text_field :office_name %></p>
      <%end%>

  <%end%>
  <p><%= f.submit 'Sign Up' %></p>
<%end%>

Darian Shimy