Thanks for the help, so I understand that it should be RegisterStartupScript(typeof(String), param2, param3) Or RegisterStartupScript(GetType("String"), param2, param3) instead of RegisterStartupScript(GetType(String), param2, param3) Appreciate the help Andy Carpe Diem

Hi there The exception might have been thrown from the OnDataBinding method, if the SelectedValue property is set before the DataBind method is called, the error as you said will be raised.