DataTable internal index is corrupted: '5'.used in threading

DataTable dt = new DataTable();
dt.Columns.Add("secuencia", typeof(System.Int32));
dt.Columns.Add("comercio", typeof(System.Int32));
dt.Columns.Add("fecha", typeof(System.DateTime));
dt.Columns.Add("valor", typeof(System.Decimal));
dt.Columns.Add("propina", typeof(System.Decimal));
dt.Columns.Add("iva", typeof(System.Decimal));
dt.Columns.Add("total", typeof(System.Decimal));
DataRow dr = null;

dr = dt.NewRow();dr["secuencia"] = 1; dr["comercio"] = 1; dr["fecha"] = DateTime.Now; dr["valor"] = 32.21; dr["propina"] = 0.12; dr["iva"] = 0.41; dr["total"] = 35.79; dt.Rows.Add(dr);
dr = dt.NewRow();dr["secuencia"] = 2; dr["comercio"] = 2; dr["fecha"] = DateTime.Now; dr["valor"] = 67.43; dr["propina"] = 0.48; dr["iva"] = 0.25; dr["total"] = 35.79; dt.Rows.Add(dr);
dr = dt.NewRow();dr["secuencia"] = 3; dr["comercio"] = 3; dr["fecha"] = DateTime.Now; dr["valor"] = 51.76; dr["propina"] = 0.54; dr["iva"] = 0.39; dr["total"] = 35.79; dt.Rows.Add(dr);
dr = dt.NewRow();dr["secuencia"] = 4; dr["comercio"] = 1; dr["fecha"] = DateTime.Now; dr["valor"] = 65.09; dr["propina"] = 0.67; dr["iva"] = 0.43; dr["total"] = 35.79; dt.Rows.Add(dr);
dr = dt.NewRow();dr["secuencia"] = 5; dr["comercio"] = 4; dr["fecha"] = DateTime.Now; dr["valor"] = 109.59; dr["propina"] = 0.91; dr["iva"] = 0.55; dr["total"] = 35.79; dt.Rows.Add(dr);
dr = dt.NewRow();dr["secuencia"] = 6; dr["comercio"] = 2; dr["fecha"] = DateTime.Now; dr["valor"] = 93; dr["propina"] = 0.84; dr["iva"] = 0.76; dr["total"] = 35.79; dt.Rows.Add(dr);
dr = dt.NewRow();dr["secuencia"] = 7; dr["comercio"] = 4; dr["fecha"] = DateTime.Now; dr["valor"] = 93.09; dr["propina"] = 0.43; dr["iva"] = 0.97; dr["total"] = 35.79; dt.Rows.Add(dr);
dr = dt.NewRow();dr["secuencia"] = 8; dr["comercio"] = 2; dr["fecha"] = DateTime.Now; dr["valor"] = 41.72; dr["propina"] = 0.97; dr["iva"] = 0.75; dr["total"] = 35.79; dt.Rows.Add(dr);
dr = dt.NewRow();dr["secuencia"] = 9; dr["comercio"] = 1; dr["fecha"] = DateTime.Now; dr["valor"] = 87.54; dr["propina"] = 0.19; dr["iva"] = 0.81; dr["total"] = 35.79; dt.Rows.Add(dr);
dr = dt.NewRow();dr["secuencia"] = 10; dr["comercio"] = 3; dr["fecha"] = DateTime.Now; dr["valor"] = 98.32; dr["propina"] = 0.27; dr["iva"] = 0.83; dr["total"] = 35.79; dt.Rows.Add(dr);
dr = dt.NewRow();dr["secuencia"] = 11; dr["comercio"] = 3; dr["fecha"] = DateTime.Now; dr["valor"] = 124.65; dr["propina"] = 0.37; dr["iva"] = 0.89; dr["total"] = 35.79; dt.Rows.Add(dr);
dr = dt.NewRow();dr["secuencia"] = 12; dr["comercio"] = 4; dr["fecha"] = DateTime.Now; dr["valor"] = 48.28; dr["propina"] = 0.62; dr["iva"] = 0.58; dr["total"] = 35.79; dt.Rows.Add(dr);

DataTable dt2 = new DataTable();
dt2.Columns.Add("secuencia", typeof(System.Int32));
dt2.Columns.Add("comercio", typeof(System.Int32));
dt2.Columns.Add("fecha", typeof(System.DateTime));
dt2.Columns.Add("valor", typeof(System.Decimal));
dt2.Columns.Add("propina", typeof(System.Decimal));
dt2.Columns.Add("iva", typeof(System.Decimal));
dt2.Columns.Add("total", typeof(System.Decimal));

ParallelOptions pOpciones = new ParallelOptions();
pOpciones.MaxDegreeOfParallelism = 4; //Maximo grado de paralelismo; en este Caso 4 threads
Parallel.ForEach(dt.AsEnumerable(), pOpciones, drow =>
{
    //*******************************************
    lock (dt2) // I use lock for every thread ;)
    //*******************************************
    {
        DataRow dr2 = dt2.NewRow();
        dr2["secuencia"] = drow["secuencia"];
        dr2["comercio"] = drow["comercio"];
        dr2["fecha"] = drow["fecha"];
        dr2["valor"] = drow["valor"];
        dr2["propina"] = drow["propina"];
        dr2["iva"] = drow["iva"];
        dr2["total"] = drow["total"];
        dt2.Rows.Add(dr2);
    }
});

string sec = "";
string valor = "";
rtbOrigen.Text = "";
foreach (DataRow fl in dt.Rows)
{
    sec = fl["secuencia"].ToString();
    if (sec.Length == 1)
        sec = " " + sec;
    valor = fl["valor"].ToString();
    if (valor.Length < 10)
        valor = new string(' ', 10 - valor.Length) + valor;

    rtbOrigen.Text += sec + " " +
                      fl["comercio"].ToString() + " " +
                      ((DateTime)fl["fecha"]).ToString("yyyy-MM-dd HH:mm:ss.fff") + " " +
                      valor + " " +
                      fl["propina"].ToString() + " " +
                      fl["iva"].ToString() + " " +
                      fl["total"].ToString() + "\r\n";
}
rtbCopia.Text = "";

//*********************************************************************
//In this foreach I get the error when I don't use the lock statement
//*********************************************************************
foreach (DataRow fl2 in dt2.Select("", "secuencia"))
{
    sec = fl2["secuencia"].ToString();
    if (sec.Length == 1)
        sec = " " + sec;

    valor = fl2["valor"].ToString();
    if (valor.Length < 10)
        valor = new string(' ', 10 - valor.Length) + valor;

    rtbCopia.Text += sec + " " +
                      fl2["comercio"].ToString() + " " +
                      ((DateTime)fl2["fecha"]).ToString("yyyy-MM-dd HH:mm:ss.fff") + " " +
                      valor + " " +
                      fl2["propina"].ToString() + " " +
                      fl2["iva"].ToString() + " " +
                      fl2["total"].ToString() + "\r\n";
}

DataTable.BeginLoadData();

// Do MultiThreaded Inserts

DataTable.EndLoadData(); 
DataTable.AcceptChanges();