Have parent state default to child state using ui-router

I have this state structure:

.state('places',
{
    url:'/places',
    controller:'PlacesController',
    templateUrl:'views/places.html'
})
.state('places.city',
{
    url:'/:city',
    templateUrl:function(stateParams)
    {
        return 'views/places/'+stateParams.city+'.html';
    }
});

It works nicely except when the URL is simply /places without a city after it.
How can I have a default value for templateUrl in the child state?

I'm thinking the best way would be to set the ui-view from PlacesController, however that doesn't seem to be possible.