Custom helper prevents value in template updating

gistfile1.js

// In controller:
MyApp.MyController = SC.Object.create({

  items: [],

  total: function(){
    // calculate the total value of items
  }.property('items.[]')

})

// A simple custom helper:
Handlebars.registerHelper('formatCurrency', function(property) {
  var value = SC.getPath(this, property);
  return "£" + Number(value).toFixed(2);
});




// Updates as items are added
{{ MyController.total }}

// Renders correctly initially but won't update as items are added
{{ formatCurrency MyApp.MyController.total }}

// I ended up having to bind to controller property that did formatting, not ideal