为什么在NpgSql中serializer.ToStream为null？

Question

我在NpgSqlCopyIn行上得到null异常。

序列化程序不是null但是ToStream是 - 为什么？

    string connstring = String.Format("Server={0};Port={1};User Id={2};Password={3};Database={4};", <removed creds>);
    NpgsqlConnection dbcon = new NpgsqlConnection(connstring);


    NpgsqlCommand dbcmd = dbcon.CreateCommand();
    string sql = "COPY MIN_BAR_LASTS FROM STDIN;";
    dbcmd.CommandText = sql;
    NpgsqlCopySerializer serializer = new NpgsqlCopySerializer(dbcon);
    NpgsqlCopyIn copyIn = new NpgsqlCopyIn(dbcmd, dbcon, serializer.ToStream);

    copyIn.Start();

    var lines = File.ReadAllLines(@"C:\PostgreSQLLoadFile.txt");
    foreach(var line in lines){
        serializer.AddString(line);
        serializer.EndRow();
        serializer.Flush();
    }
    copyIn.End();
    serializer.Close();