error output

Uncaught TypeError: Object #<FormTemplate> has no method 'apply'               backbone.js:98
Backbone.Events.trigger                                                        backbone.js:98
_.extend.set                                                                   backbone.js:195
_.extend.fetch.options.success                                                 backbone.js:257
jQuery.jQuery.extend._Deferred.deferred.resolveWith                            jquery-1.5.js:864
done                                                                           jquery-1.5.js:6480
jQuery.ajaxTransport.send.callback                                             jquery-1.5.js:7251

file_template.coffee

class FormPage extends Backbone.Model
  defaults:
    label: 'New Page'
    position: 0
    items: []

class FormPageCollection extends Backbone.Collection
  model: FormPage

class FormTemplate extends Backbone.Model
  defaults:
    version: ''
    pages: []

  constructor: ->
    super
    @bind 'change:pages', @_initializePages()
    @activePage = @get('pages').first()

  _initializePages: ->
    if _( @get 'pages' ).isEmpty()
      @set { pages: FormPageCollection }
      newPage = new FormPage
      @get('pages').add newPage
    else
      @set { pages: new FormPageCollection @get('pages') }