I've been staring at this for a while and trying various tweaks, to no avail. 

Why am I getting a "this.model is undefined" error at 

```
$(function(){

  window.Sentence = Backbone.Model.extend({ 

    initialize: function() {
      console.log(this.toJSON()) 
    }
  });

  window.Text = Backbone.Collection.extend({
    model : Sentence,

    initialize: function(models, options){
      this.url = options.url;
    }
  });

  window.SentenceView = Backbone.View.extend({
    initialize : function(){
      _.bindAll(this, 'render');
      this.template = _.template($('#sentence_template').html());
    },

    render : function(){
      var rendered = this.template(this.model.toJSON());
      $(this.el).html(rendered);
      return this;
    }
  })

  window.TextView = Backbone.View.extend({
    el : $('#notebook') ,

    initialize : function(){
      _.bindAll(this, 'render');
    },

    render : function(){
      this.collection.each(function(sentence){
        if (sentence === undefined){
          console.log('sentence was undefined');
        };

        var view = new SentenceView({model: sentence});
        this.$('ol#sentences').append(view.render().el);
      });

      return this;
    }
  });

  function Notebook(params){
    this.text = new Text(
      // models
      {}, 
      // params
      {
        url: params.url 
      }
    );

    this.start = function(){
      this.text.fetch();
      this.textView = new TextView({
        collection: this.text
      });
      $('body').append(this.textView.render().el);
    };
  }

  window.notebook = new Notebook(
    { 'url': 'js/mandarin.js' }
  );

  window.notebook.start();

})

```

There's an online version wher eyou can see the error in a console at:

<http://lotsofwords.org/languages/chinese/notebook/> 

The whole repo is at:

<https://github.com/amundo/notebook/>

The offending line appears to be at:

<https://github.com/amundo/notebook/blob/master/js/notebook.js#L31>

I find this perplexing because as far as I can tell the iteration in `TextView.render` has the right `_.each` syntax, I just can't figure out why the `Sentence` models aren't showing up as they should.