Consuming a db table from Silverlight app using a service

Why does this not work:

[OperationContract]
public List<Category> DoWork()
{
    using(var db= new PDataContext())
    {
        return db.Categories.Select(x => x).ToList();
    }
}

I get a CommunicationException error: Not Found.

But this does:

[OperationContract]
public List<myCategory> DoWork()
{
    using(var db= new PDataContext())
    {
        return  db.Categories.Select(x => new myCategory
                {
                  CategoryID = x.CategoryID,
                  Name = x.Name,
                  Visible = x.Visible,
                  ParentID = x.ParentID
                 }).ToList(); 
    }
 }

 public class myCategory
 {
     public int CategoryID { get; set; }
     public string Name { get; set; }
     public bool Visible { get; set; }
     public int ParentID { get; set; }
 }

I'm failing to understand how the LINQ2SQL Category class is in any way different than the code I wrote in the example above.