Pop up with a random string of an array

I'm trying to make an alert box pop up with one random sentence from an array of sentences inside it. This is my code:

var tasks = [
  "This is the first task",
  "And this is the second task",
  "Third task..."
];

var randomTask = Math.floor((Math.random() * tasks.length) - 1);

alert(tasks[randomTask]);

If you run it, the only thing the pop says is "undefined". Why doesn't it work?

Thanks to anyone that answers! :-)