It's not only iPhone

You have to select "ProfStef next." text on the page, in a manner browser's document.getSelection would find those (not sure whenever this is possible on iPhone).

I think, given this is a newbie-oriented tutorial, the alert should be more helpful, like "empty program, select some text first!" instead of unexpected null character.

I didn't check it from iPhone. From my desktop (iMac) I only place the cursor on the "ProfStef next." line and press Ctrl-d or press the DoIt or PrintI - I didn't need to select the text and everything worked.

You have to select the text and hit the button.