sessionStorage.setItem("points", points); - children could not be evaluated

I like to store some Data on client side. something like

 var points = [];

for (i = 0; i < 50; i += 1) {
            points.push({ x: Math.random() * 100,
                y: Math.random() * 100,
                vx: Math.random() * 10 - 5,
                vy: Math.random() * 10 - 5
            });

sessionStorage.setItem("points", points);

var retreivepoints = sessionStorage.getItem("points");   -  children could not be evaluated

Thanks for any sugestion