Razor: Declarative HTML helpers

I'm trying to write a simple declarative html helper:

@helper Echo(string input) {
    @input
}

The helper works fine if I embed it into the page I want to use it on. But if I move it to a separate .cshtml file and place that file in the ~/Views/Helpers directory, my view can't be compiled anymore because the helper is not found. According to Scott Gu's blog article on Razor it should work.

What am I doing wrong?