Data Controls :: Cannot Set Column Excess

I am having a problem editing data on a datatable . I have loaded data into a datatable and some columns got null value which i placed that in an on the SP now I want to edit some columns in the datatble

dt1 = AuthWorkdone.View_AuthorsationReport_WorkDone1(DateTime.Parse(txtStart.Text.Trim()), DateTime.Parse(txtEnd.Text.Trim()));
if ( dt1.Rows.Count >0 ) {
foreach(DataColumn col1 in dt1.Columns) {
col1.ReadOnly = false; }

[code]...

the column "Excess" in the table is varchar.i am getting the following error on this line of code Cannot set column 'Excess'. The value violates the MaxLength limit of this column.

row1.SetField("Excess",dtAuthdetails1.Rows[0][3].ToString().ToString());

The columns has numbers like 0.00 or 500.00 and also words like TBA and ect