Object reference not set to an instance of an object on GridView Button Edit click

i m using the following code to change the back color of cell but vaue of e.Row.Cells(4) is coming as nothing

Protected Sub OnRowDataBound(sender As Object, e As GridViewRowEventArgs)
   If e.Row.RowType = DataControlRowType.DataRow Then
      Dim cell As TableCell = e.Row.Cells(4)
      Dim quantity As String = (cell.Text).ToString

      If quantity < "10000" Then
         cell.BackColor = Color.Red
      End If
      If quantity >= "10000" AndAlso quantity < "50000" Then
         cell.BackColor = Color.Yellow
      End If
      If quantity >= "50000" Then
         cell.BackColor = Color.Orange
      End If
   End If

End Sub

is i take quantity as integer it is giving system.format.exception