combobox problem

ddDate is a RadCombobox and that line starts with ddDate is giving error saying invalid argument about adding items to combobox..
 DateTime dateCounter = System.DateTime.Now;
            System.DateTime today = System.DateTime.Now;

            while (dateCounter >= today.AddDays(-30))
            {
                 
                ddDate.Items.Add(new ListItem(dateCounter.ToString("d"), dateCounter.ToString("d")));
                dateCounter = dateCounter.AddDays(-1);
            }