I've got a hidden form field, and when a button gets pressed the value of the hidden field is changed. Now, I've added an observer to the hidden field, listening for changes to occur. For some reason, though, the event listener never kicks in, even though the value of the hidden element changes. I'm using Prototype and Firefox 3.6.

The code looks roughly like this:

```
button.observe('click', function(event) {
  hiddenField.setValue(someValue);
});

hiddenField.observe('change', function(event) {
  alert('It works!');
});

```

Does anyone have a clue why the latter observer doesn't execute?

Thanks!