jit.qt.record and javascript

I'm trying this out and I'm not sure how to stop a recording!

var rec = new JitterObject("jit.qt.record");
var path = "mydrive:/movies/movie.mov";
rec.write(path,30,"jpeg","normal",600);

will create a file, but if I have

rec.matrixcalc(outmatrix,outmatrix);

as part of my process and then call a function with

rec.stop();

it doesn't seem to stop recording and close the file, though it provides no error. The file always stays at 0k, and claims not to be a movie file when I open it in QT Player.

What am i missing?