Linq to sql: Composite foreign key in one-many relation?

Question

Hi,

I'm trying to model a database in LINQ to SQL. I have a table JournalPosts. Each entity is associated with 1 or more senders or recipients. I have the following composite primary key in my table "JournalPosts":

    [Column(IsPrimaryKey = true)]
    public int CaseYear { get; set; }

    [Column(IsPrimaryKey = true)]
    public int CaseSequenceNumber { get; set; }

    [Column(IsPrimaryKey = true)]
    public int DocumentNumber { get; set; }

I define the one-many relationship like this in JournalPosts:

    private EntitySet<SqlSenderRecipient> senderRecipients;

    [Association(OtherKey = "CaseYear, CaseSequenceNumber, DocumentNumber", Storage="senderRecipients")]
    private EntitySet<SqlSenderRecipient> SenderRecipients
    {
        get { return senderRecipients; }
        set 
        { 
            senderRecipients.Assign(value); 
        }
    }

In my other table (SenderRecipients) I've tried to define the fields like this:

    [Association(IsForeignKey=true)]
    public int CaseYear { get; set; }

    [Association(IsForeignKey=true)]
    public int CaseSequenceNumber { get; set; }

    [Association(IsForeignKey=true)]
    public int DocumentNumber { get; set; }

The problem is, I get a "Object reference not set to an instance of an object." error when I try creating the database. However, if I specify the Column attribute in the other table (SenderRecipients) istead of Association-attributes, it works (but then the columns isn't set as foreign keys in the database - which is what I want).

Do anyone know how to achieve what I want?

Thanks in advance.