Generate artboards

Hi, why do I get  an error on the following sccript?

#target illustrator

var _doc = app.activeDocument;

var _msg = "Generate artBords";

var _count = Number(Window.prompt ("Enter Number of artboards max 100", 10, _msg));

var _size = Number(Window.prompt ("Enter artboard Size ", 50, _msg));

var _margin = Number(Window.prompt ("Enter artboard Margins ", 10, _msg));

var _artBoards =_doc.artboards;

     for ( var i = 0  ; i < _count  ; i++){

    var _newRect = [];

    _newRect[0] =   i * ( _size + _margin );

    _newRect[1] = Math.floor(i/5) * ( _size + _margin );

    _newRect[2] = _newRect[0] + _size;

    _newRect[3] = _newRect[1] + _size;

  $.writeln("newArtBoard: " + _newRect);

//var _newArtBoard = _artBoards.add(_newRect);

app.activeDocument.artboards.add( _newRect );

}