datagrid column problem

[Bala]

Hi,
i added the column in datagrid like the below code. then when i try to add
row using this code it says "Column 'ImpNo' does not belong to table ". whats
the problem?

create a column code:
dt.Columns.Add(New DataColumn("ImpNo", GetType(Integer)))
dt.Columns.Add(New DataColumn("WPI", GetType(String)))

value insert code
r = Tbl.NewRow()
r("ImpNo") = tmptxtDisabilityNo
r("WPI") = tmptxtstdNo

thanks
bala