Why the result is empty?

var normal = new CSG.Vector(0, 0, 0);
var v1 = new CSG.Vertex(new CSG.Vector(0, 0, 0), normal);
var v2 = new CSG.Vertex(new CSG.Vector(0, 1, 0), normal);
var v3 = new CSG.Vertex(new CSG.Vector(1, 1, 0), normal);

var polygon1 = new CSG.Polygon([v1,v2,v3], false);
var csg1 = CSG.fromPolygons([polygon1]);

var v4 = new CSG.Vertex(new CSG.Vector(0, 0, 0), normal);
var v5 = new CSG.Vertex(new CSG.Vector(1, 0, 0), normal);
var v6 = new CSG.Vertex(new CSG.Vector(1, 1, 0), normal);

var polygon2 = new CSG.Polygon([v4, v5, v6], false);
var csg2 = CSG.fromPolygons([polygon2]);
var result = csg1.union(csg2);