Nested form mass assignment error with a has many through relationship

Question

I'm trying to use a nested form at the moment to add category tags to a song as you create the song. At the moment it's throwing a mass assignment error every time I submit the form despite the fact that I've put in what I believe to be the correct attribute accessible characteristics. Obviously I've gone wrong somewhere so it'd be great if someone could point that out for me. The form is:

<%= nested_form_for(@song) do |f| %>
         ...
      <%= f.fields_for(@song.categorizations.build) do |cat| %>
        <%= cat.label :category_id, "TAG" %>
        <%= cat.select :category_id, options_for_select(Category.all.collect {|c| [ c.tag, c.id ] }, 
        { :include_blank => true }), prompt: "" %>
      <%end%>
      <%= f.submit "Save" %>
    <% end %>

The relevant model here is:

class Song < ActiveRecord::Base
  attr_accessible :artist, :title, :categorizations_attributes
  has_many :categorizations, dependent: :destroy
  has_many :categories, through: :categorizations

  accepts_nested_attributes_for :categorizations

The song controller looks like this:

 def create
    @song = Song.new(params[:song])
    if @song.save
        flash[:success] = "Song successfully added to library"
        redirect_to @song
    else
        #FAIL!
        render 'new'
    end
  end

Finally the error being raised is:

ActiveModel::MassAssignmentSecurity::Error in SongsController#create

Can't mass-assign protected attributes: categorization

Thank you in advance for any help!