OleDbDataReader.GetString异常-指定的强制类型转换无效

当我的c# (. NET 3.5)应用程序试图访问MS access 2007数据库时，OleDbReader.GetString()方法抛出异常:

指定的类型转换无效。

我做错了什么?

OleDbCommand cmd = null;
OleDbDataReader reader = null;
String queryString = "SELECT ids.ENUM_H, bas.[BAS BACnet Object Type/Instance] FROM [OV2 BAS] AS bas INNER JOIN [OV2 RefID] AS ids ON bas.[Ref ID] = ids.[Ref ID]";
this.Open();
try
{
    cmd = new OleDbCommand(queryString, this._conn);
    reader = cmd.ExecuteReader();
    if (!reader.HasRows)
    {
        Exception e = new Exception("Read of mapping table returned no results.");
        throw e;
    }
    else
    {
        while (reader.Read())
        {
            Int32 index;
            String classTypeString = null; // = reader.GetString(reader.GetOrdinal(MappingTable.OBJECT_IDENTIFIER_COLUMN_NAME)).Substring(0, 2);
            int it = reader.GetOrdinal(MappingTable.OBJECT_IDENTIFIER_COLUMN_NAME);
            string st = reader.GetString( it );  // <-- **Exception is thrown here** <--
            st = st.Substring(0,2);
            String classIdString = reader.GetString(reader.GetOrdinal(MappingTable.OBJECT_IDENTIFIER_COLUMN_NAME)).Substring(2);
            index = Convert.ToInt32(classIdString);
            ClassIds[index, 0] = reader.GetString(reader.GetOrdinal("ENUM_H"));
            ClassIds[index, 1] = classTypeString;
        }
    }
}
catch (Exception e)
{
    Console.WriteLine("ERROR: " + e.Message);
    Console.WriteLine(e.ToString());
    throw e;
}
this.Close();

我知道Open()和Close()方法是有效的。我的查询或我处理结果的方式有问题。谢谢。