Exception deserializing JSON object with custom dictionary/collection

Hi,
I have a class the has a property that is a custom collection:
public class Foo
{
      :
      public NoteDictionary Notes
      {
            get { return _notes; }
            set { _notes = value; }
      }
      :
}
public class NoteDictionary : System.Collections.Generic.Dictionary<string, Note>
{
      public NoteDictionary()
      {
      }
      :
}
 
When I call the web service using Ajax.net:
[WebMethod]
public object Save(Foo foo)
{
  &nbs ...