Event binding doesn't seem to work

Polygons that are overlaid appear as clickable but I can't seem to add click events using the google maps API:

``` js
overlay = new GeoJSON( data );
google.maps.event.addListener( overlay, 'click', function( e ) {
   console.log( e );
} );
```

I can't work out if there's a sub object I need to bind to. Nothing I've tried seems to bind the event.
