DropDownList从数据库读取数据后,不显示第一个,该怎样解决啊?

string sqlstr = "select * from Unit";
        try
        {
            con = new SqlConnection(strsqlcon);
            con.Open();
            cmd = new SqlCommand(sqlstr, con);
            reader = cmd.ExecuteReader();
            ddlweightunit.Items.Clear();
            while (reader.Read())
            {
                ddlweightunit.Items.Add(new ListItem ( reader["UnitName"].ToString(),""));
            }
            reader.Close();
        }
        catch (Exception ee)
        {
            Response.Write(ee.ToString());
        }
        finally
        {
            con.Close();
        }