DataGridのセルの色指定時の例外エラー

public class DataGridBackBrush : DataGridTextBoxColumn

{

  protected override void Paint(Graphics g, 

                                Rectangle bounds, 

                                CurrencyManager source,

                                int rowNum,

                                Brush backBrush,

                                Brush foreBrush,

                                bool alignToRight)

  {

     object cellValue = this.GetColumnValueAtRow(source, rowNum);

     if ((int) cellValue < 0){

       backBrush = new SolidBrush(Color.Red);

     }else if ((int) cellValue >= 0 | (int) cellValue <= 5){

       backBrush = new SolidBrush(Color.Yellow);

     }

     base.Paint(g, bounds, source, rowNum,backBrush, foreBrush, alignToRight);

  }

}





呼び出し側は

DataGridTableStyle ts = new DataGridTableStyle();

ts.MappingName = "table1";



DataGridBackBrush cs;

cs = new DataGridBackBrush();

cs.MappingName = "field1";

cs.HeaderText = "*****";

ts.GridColumnStyles.Add(cs);



ts.GridLineColor = Color.BlueViolet;

dataGrid1.TableStyles.Add(ts);