If I create a cylinder geometry by doing
var cyl:CylinderGeometery = new cyl:CylinderGeometery()
cyl.topRadius = 5; // Or any other property change

Then I see the correct shape, but I also see an artifact that looks like a piece of the side of the original cylinder. 

But if I pass in all the properties when I create the cylinder, e.g.
var cyl:CylinderGeometery = new cyl:CylinderGeometery(5, etc)

then it renders correctly.  Is this a bug, or do I need to do something to clear the geometry before or after I change its properties?
Thanks!