DOM - getElementById()

[Matze]

Wieso funktioniert anweisung im switch-block nicht???

<form name="wallpaper">
<script type="text/javascript">
switch(screen.height) {
case 960: document.getElementById("res").selectedIndex = 3;
   break;
};
</script>
Wallpaper in
<select name="res" id="res">
<option value="800600">800x600</option>
<option value="1024768">1024x768</option>
<option value="1152864">1152x864</option>
<option value="1280960">1280x960</option>
<option value="12801024">1280x1024</option>
<option value="16001200">1600x1200</option>
</select>
 Pixeln
<input name="walldown" type="button" value="downloaden." />
</form>

schon mal danke.
Grüße...