sharePoint Linq error

 when i wtite this sharepoint linq query 

var Announcement = (from cust in dataContext.Announcements.ToList()
                                where
                                 (cust.AnnouncementType.AnnouncementType == "External")
                                ||
                                (cust.AnnouncementType.AnnouncementType == "Both")
                                 &&
                                  cust.ISHidden == false
                                orderby cust.PriorityNumber
                                select
                                new

                                {
                                    cust.Id,
                                    cust.PictureURL,
                                    cust.AnnouncementType.AnnouncementType
                                });

i get this error

At least one object must implement IComparable.

Nour