ASP.net dropdownlist

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ASP.NET

I am Binding table to dropdownlist with datasource property and displaying
records in gridview.
I want to display selected record in grid to respective fields in page
It works fine with Textfield but gives error with dropdownlist
error:'DropDownList' has a SelectedValue which is invalid because it does not exist in the list of items'

//Combo();
       int id  = Convert.ToInt32(((Label)gvEmp.SelectedRow.Cells[1].FindControl("lblId")).Text) ;
       txtName.Text= ((Label)gvEmp.SelectedRow.Cells[2].FindControl("lblName")).Text;
       string desig;
       desig = gvEmp.SelectedRow.Cells[3].Text.ToString();
       ddlDesig.ClearSelection();
       ddlDesig.Text = desig;

answer appreciated