LINQ to SQL - Null value cannot be assigned to a member with type System.Int64 which is a non-nullable value type

I am attempting an InsertOnSubmit and it is failing since "The null value cannot be assigned to a member with type System.Int64 which is a non-nullable value type" This LINQ to SQL is driving me crazy. Please help.

The table in the SQL database has a MessageID column which is a BigInt with primary key (NOT NULL IDENTITY 1 1) no Default.

The field looks like this in the designer file to the dbml:

[global::System.Data.Linq.Mapping.ColumnAttribute(Storage="_MessageId", AutoSync=AutoSync.OnInsert, DbType="BigInt NOT NULL IDENTITY", IsPrimaryKey=true, IsDbGenerated=true)] 
public long MessageId 
{ 
    get 
    { 
        return this._MessageId; 
    } 
    set 
    { 
        if ((this._MessageId != value)) 
        { 
            this.OnMessageIdChanging(value); 
            this.SendPropertyChanging(); 
            this._MessageId = value; 
            this.SendPropertyChanged("MessageId"); 
            this.OnMessageIdChanged(); 
        } 
    } 
}

So the problem is that null cannot be assigned... right. But I'm not passing null! How can I be? I'm using a long (a value type so can't be null by nature).

One workaround I found was making this field nullable but this feels like a hack since it should never be null. Anyway this just moves the problem to the ID not AutoSyncing OnInsert. (AutoSync=OnInsert is set).

Update and solution

Well it took me a while, I even posted on stackoverflow to no avail but finally worked out the problem and wrote about it in this post about optimistic concurrency exceptions on insert although by this time it has manifested itself in a new way since I'm using Entity Framework.