Why createVideoTexture() can return null?

Hello everyone! Why createVideoTexture() can return null.

code:

            var nc:NetConnection;

            var ns:NetStream;

            nc = new NetConnection();

            nc.connect(null);

            ns = new NetStream(nc);

            ns.play("red_galaxy.mp4");

            var context3D:Context3D = Starling.context;

            var texture:VideoTexture;

            texture = context3D.createVideoTexture(); //null

I read the guide, but it did not help.