Allow multi selection mode in AW.UI.Combo

Hello,

Using Firefox 3.5.7 and AW 2.5.5, I am trying to allow multi-select on an AW.UI.Combo box.

Looking at the following code based on code from one of the examples...

var obj = new AW.UI.Combo;
obj.setControlText("Combo");
obj.setControlImage("favorites");
obj.setItemText(["Home", "Favorites", "Font size", "Search"]);
obj.setItemImage(["home", "favorites", "fontsize", "search"]);
obj.setItemCount(4);
obj.setSelectionMode("multi"); //according to the documentation this should be all I need, afaik
document.write(obj);

Despite having...

obj.setSelectionMode("multi");

...the combo box still behaves as in single-selection mode.

Am I missing something obvious? I can't find any specific examples of this, but it would be really handy to have.