Error on Parameterized Queries using Enterprise Library 4.0

I'm creating a simple Data Access Layer for an app using the Enterprise Library 4.0. I can execute simple sql queries and they return data. However, when I try to create a "Parameterized Query"  the following exceptio occurs.

System.Data.SqlClient.SqlException was caught
  Message="Parameterized Query '(@symbol_id int)Select * from Symbol where [symbol_id] = @symbol' expects parameter @symbol_id, which was not supplied."
  Source=".Net SqlClient Data Provider"

The parameter never gets set and I can't see why!!
 

private static readonly string SQL_SELECT = @"Select * from Symbol where [symbol_id] = @symbol_id";
        
        public IDataReader Select(System.Nullable<int> id)
        {
            try
            {
                Database db = DatabaseFactory.CreateDatabase("myConnection");
                DbCommand cmd = db.GetSqlStringCommand(SQL_SELECT);
                cmd.Parameters.Add(new SqlParameter("@symbol_id", SqlDbType.Int));
                cmd.Parameters["@symbol_id"].Value = id;
                return db.ExecuteReader(cmd);
            }
            catch
            {
                throw;
            }
        }

Select allOpen in new window