error is not a constructor - firefox

Question

I am new to backbone and requirejs...
I am working on a small gallery app where i came across this error in firefox firebug and in chrome it doesn't show any error chrome dev tool... please help me in this
here is my code
PhotosCollectionjs
define([
'jquery',
'underscore',
'backbone',
'models/thumb/PhotoModel'
], function ($, _, Backbone, PhotoModel) {
var PhotosCollection = Backbone.Collection.extend({
model: PhotoModel,
url: '/photos?query=xxx',
fetch: function (options) {
options = options || {};
.......
Backbone.Collection.prototype.fetch.call(this, options);
},
parse: function (response) {
......
return response.photos;
}
});
return PhotosCollection;
});
loadphotos js
define([
'jquery',
'underscore',
'backbone',
'collections/thumb/PhotosCollection'
], function ($, _, Backbone, PhotosCollection) {
console.log(PhotosCollection); .... undefined(firebug) ... function (){return r.apply(this,arguments)} (chrome dev tool)
var loadPhotos = function (container, options) {
var photos;
photos = void 0;
if (options) {
photos = new PhotosCollection();
options.collection = photos;
}
.....................
................
};
return loadPhotos;
});
in firebug it is giving error
TypeError: PhotosCollection is not a constructor
photos = new PhotosCollection();
EDIT
its temporarily solved .... but now again after few refreshes i receive same error in firebug but no error in chrome... Please someone help me
Edit 2
Trial and error i found that it occurs only while debugging with breakpoints.... if i remove all breakpoints its giving no error... is it related to async loading of scripts or something else???

Answer 1

i just included initialize in PhotosCollection js and now its giving no error... i dont know what exactly the problem but it got solved.....
But i am interested in knowing the cause for it... as only firebug thrown error and chrome doesn't... Please give me some insight
here is my new photosCollection js with blank intialize....
define([
'jquery',
'underscore',
'backbone',
'models/thumb/PhotoModel'
], function ($, _, Backbone, PhotoModel) {
var PhotosCollection = Backbone.Collection.extend({
model: PhotoModel,
url: '/photos?query=xxx',
initialize:function(options){},
fetch: function (options) {
options = options || {};
.......
Backbone.Collection.prototype.fetch.call(this, options);
},
parse: function (response) {
......
return response.photos;
}
});
return PhotosCollection;
});