primary key error

Question

At first I have to find a row and then I have to change it. So I use Rows.Find() method for finding specific row and then I set its 6. col with this code this.dSet.Tables[0].Rows.Find( txt1.text.Trim() )[5] = this.txt5.Text.Trim(); but it gave this error. An unhandled exception of type 'System.Data.MissingPrimaryKeyException' occurred in system.data.dll Additional information: Table doesn't have a primary key. But Tables[0] have primary key. Why it gave this error? pls help