"No Such Element". Intro to scripting.

Okay...

  Here's my basic script. Cut and pasted from Adobe's Intro to Scripting PDF for CS5. I run it and it produces a "No such element" error in line #2. I want to understand why the very first script that Adobe tells me to use in its Intro to Scripting tutorial does not work properly. Is this my fault? Is there something they assume I already know that leads this script to fail?

thanks.

var myDoc = app.documents.add()

var myLayer = myDoc.layers.add()

var myTextFrame = myLayer.textFrames.add()

     myTextFrame.contents = "Hello world!"