System.FormatException

I am work on a silverlight app which use a json data.

I am using System.Json to work with a Json file. I am using Stream to construct the JsonObject for the app. But it always keeps throwing the exception System.FormatException.

Here is the code,

Stream fs = File.Open(new MainWindow().getFileName("events"), FileMode.Open);

            JsonObject jsonObject = (JsonObject) JsonObject.Load(fs);

any ideas?