Backbone.js bootstrap with an array of JSON objects

I am attempting to bootstrap a backbone collection using an array of JSON objects like in the code below. However, when I try and call reset on the collection object I get an error from backbone - Uncaught TypeError: undefined is not a function.

If I change the JSON array to an array of Users.UserModel objects then it works. I must be missing something fundamental in the collection initialization method or something similar as all the examples I have seen don't really contain much code than the call to reset.

class Users.UsersCollection extends Backbone.Collection
    model: Users.UserModel
    url: '/Users'

class Users.UserModel extends Backbone.Model

# document ready
$ ->
    Users.userCollection = new Users.UsersCollection()

    users = [
        { Id: 1, Username: 'dan', FirstName: 'Dan', LastName: 'Ormisher' },
        { Id: 1, Username: 'simon', FirstName: 'Simon', LastName: 'Lomax' },
        { Id: 1, Username: 'jon', FirstName: 'Jon', LastName: 'Swain' },
        { Id: 1, Username: 'martin', FirstName: 'Martin', LastName: 'Rue' }
    ]

    Users.userCollection.reset(users)

(I'm using coffeescript btw, but that is irrelevant)