This should probably be just `res` because while the second argument in a `success` callback is `res.body` in Backbone, the second argument in an `error` callback is the xhr object (the equivalent being the `res` object here). `res.body` would be the equivalent of getting `JSON.parse(xhr.responseText)`.

![image](https://f.cloud.github.com/assets/555859/1221528/b32f101e-26ed-11e3-8273-b736c565040d.png)