Problem with LINQ in C#

I am encountering a problem when using LINQ in C#, I am constantly getting "Specified cast is not valid". This is what I am trying to do.

I create a class in which I declare all the columns of the table.

    [Table(Name="tbl_Aff")]
    public class Affiliate 
    {
        [Column]
        public string name;
        [Column]
        public string firstname;
        [Column]
        public string surname;
        [Column]
        public string title;
    }

I then declare a strongly typed DataContext in which I declare all Table collections as members of the context.

    public partial class Database : DataContext 
    {
        public Table<Affiliate> affiliate;

        public Database() : base(Settings.getConnectionString()) { } //This method gets the connection string by reading from an XML file.
    }

public partial class Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            Database database = new Database();

            try
            {
                var q = from a in database.affiliate
                        select a;

                foreach (var aff in q) // Here I get the error "Specified cast is not valid"
                {
                    lblMessage.InnerHtml += aff.name + "
";
                }
            }
            catch (Exception ex)
            {
                System.Console.WriteLine(ex.Message);
            } 
        }
    }