FileStream writing strange characters

I'm trying to write out some Spanish text using FileStream(AIR), but special Spanish characters are writing out strange.

Here's a stripped down basic version of the problem, for you to try at home...

import flash.filesystem.*;
import flash.net.FileFilter;
import flash.events.Event;
var fileToSave:File = new File();
fileToSave.browseForSave("Save");
fileToSave.addEventListener(Event.SELECT, fileSaveSelected);

function fileSaveSelected(event:Event):void {
     var stream:FileStream = new FileStream();
     stream.open(event.target as File, FileMode.WRITE);
     stream.writeUTFBytes("crepúscolo");
     stream.close();
}

This should write out the word "crepúscolo", instead it writes out "crepúscolo"...

I've tried other combinations, for example i've tried stream.writeMultiByte with a variety of charsets, sometimes giving different results, but never the desired result...

Ideas anyone?