Alloy and BackBone model.save

Hi, I am a little bit confused. I am trying to use something like this:

// Since the urlRoot attribute is defined, all HTTP commands are to /library
var Book = Backbone.Model.extend({urlRoot:'/library'})
var book = new Book();
 
// Performs a POST on /library with the arguments as a payload and the server returns the id as 1
book.save({title:'Bossypants',author:'Tina Fey',checkout:false}) 
 
// Performs a GET on /library/1
book.fetch({id:1}); 
 
// Performs a PUT on /library/1 with the entire modified object as a payload.
book.save({checkout:true});
 
// Performs a DELETE on /library/1
book.destroy();

...
 line = 1359;
[ERROR] :      message = "'undefined' is not an object (evaluating '$.ajax')";
[ERROR] :      name = TypeError;
[ERROR] :      sourceId = 294813632;
...