summing columns error

Question

when i use this code vs shows following error : Specified cast is not valid. with title : InvalidCastExeption was unhandled

what is incorrect i think it must be work very good. cant find any mistake

  private void TradeLog_Load_1(object sender, EventArgs e)
        {
sqlDataAdapter1.Fill(dataSet11);
decimal allwin;
            decimal win = 0;
            decimal self;

            for (int i = 0; i < DgvLog.Rows.Count; i++)
            {
                self = (decimal)DgvLog.Rows[i].Cells["selfPriceDataGridViewTextBoxColumn"].Value;
                win += self;
            }

            allwin = sum - win;
            label7.Text = win.ToString();
        }