CEP registering event handlers

Hi,

I'm trying to register an event handler for `selectionsChanged` event.

main.js has

csInterface.evalScript('init()');

index.jsx has

function handleSelection(event) {
    if((event.type == 'selectionsChanged') && (event.object instanceof Thumbnail)) {
        alert("selections changed!");
        return {handled:false};
    }
}

function init() {
  app.eventHandlers.push( {handler: handleSelection} );
}

but the handler is never run. Why does this not work with CEP extension?

Thanks!