执行后提示:阅读器关闭时 Read 的尝试无效。

SqlConnection conn = Class1.CreateSql();
            conn.Open();
            string sql = "select VoteID,VoteTitle,VoteSum from VoteMaster";
            SqlCommand cmd = new SqlCommand(sql, conn);
            SqlDataReader sdr = cmd.ExecuteReader();
            while (sdr.Read())
            {
                Response.Write(sdr.GetString(1) + "—" + sdr.GetInt32(2) + "<br>");
                if (!sdr.IsClosed)
                {
                    sdr.Dispose();
                }

                string sql1 = "select MasterID,MasterItem ,Num from Master";
                SqlCommand cmd1 = new SqlCommand(sql1, conn);
                SqlDataReader sdr1 = cmd1.ExecuteReader();
                while (sdr1.Read())
                {
                    Response.Write(sdr1.GetString(1) + "—" + sdr1.GetInt32(2) + "<br>");
                }
        }
        conn.Close();
执行后提示:阅读器关闭时 Read 的尝试无效。
是在哪里错了..