BackboneJS + RequireJS: Undefined is not a function - backbone.js

Question

I'm trying to implement RequireJS in my project and am running into some problems. I'm getting an error is my Item model. In the model, I'm trying to set an attribute to a new collection. In that line I get the following error: "Uncaught TypeError: undefined is not a function."
My router looks something like this:
define(['backbone', 'collections/items', 'views/itemsView'], function (Backbone, Items, ItemsView) {
var Workspace = Backbone.Router.extend({
routes: {
'*path': 'nearby'
},
nearby: function() {
var items = new Items();
items.fetch();
var itemsView = new ItemsView({
collection: items
});
});
return Workspace;
});
Collection:
define(['backbone', 'models/item', 'tastypie'], function(Backbone, Item) {
var Items = Backbone.Collection.extend({
url: function() {
var url;
url = '/api/v1/nearby/?lat=' + lat1 + '&lng=' + lon1;
return url;
},
model: Item,
});
return Items;
});
Model:
define(['backbone', 'collections/locations'], function (Backbone, Locations) {
var Item = Backbone.Model.extend({
urlRoot: '/api/v1/item',
url: function () {
// stuff here
},
set: function (attrs) {
// Making a new Locations collection ordered by distance
if (attrs.items && attrs.items.length > 0) {
// Sets the collection as an attrubute
// The following line is where I get the error
attrs.locations = new Locations(attrs.items);
Backbone.Model.prototype.set.apply(this, [attrs]);
// Sets the closest attribute
attrs.closest = this.get('locations').getClosestItem().get('distance');
Backbone.Model.prototype.set.apply(this, [attrs]);
attrs.numItems = this.get('items').length;
Backbone.Model.prototype.set.apply(this, [attrs]);
}
return Backbone.Model.prototype.set.apply(this, [attrs]);
},
});
return Item;
});
Anyone know why I'm getting the error? I made sure to define to Location collection in my model.
edit: adding in the locations.js file:
define(['backbone', 'models/location'], function (Backbone, LocationModel) {
var Locations = Backbone.Collection.extend({
model: LocationModel,
comparator: function(item) {
return item.get('distance') + ' ' + item.get('name');
},
getClosestItem: function() {
return this.at(0);
}
});
return Locations;
});
Thanks for the help.