Handlebar helper inside {{#each}}

I try to call a registered handlebar helper inside a {{#each}} loop. Unfortunately Ember.js complains because it tries to resolve the helper as a property of the controller rather than a helper.

Handlebars.registerHelper('testHelper', function(name) {
    return 'foo: ' + name
});

(names and content are just dummy values to show the example)

{{#each entry in App.testController}}
   <div>{{{testHelper entry.name}}}</div>
{{/each}}

The error that the Ember.js prints is:

Uncaught Error:  Handlebars error: Could not find property 'testHelper' on object <App.testController:ember254>.

How do I need to call the registered helper so that it gets recognized?