Exception при добавлении двух осей

     public ChartWindow()
        {
            InitializeComponent();

            ChartArea ca = new ChartArea();

            // remove default Y axis
            ca.YAxises.RemoveAt(0);

            ChartAxis y1 = new ChartAxis();
            y1.Id = "Y1";

            ChartAxis y2 = new ChartAxis();
            y2.Id = "Y2";

            ca.YAxises.Add(y1);
            ca.YAxises.Add(y2);

            ChartIndicatorElement sma1 = new ChartIndicatorElement
            {
                Title = "sma1",
                Indicator = new SimpleMovingAverage { Length = 1, Name = "SMA 1" },
            };
            sma1.YAxisId = "Y1";
            ca.Elements.Add(sma1);

            ChartIndicatorElement sma2 = new ChartIndicatorElement
            {
                Title = "sma2",
                Indicator = new SimpleMovingAverage { Length = 2, Name = "SMA 2" },
            };
            sma2.YAxisId = "Y2";
            ca.Elements.Add(sma2);

            Chart.Areas.Add(ca);
        }