Helper - Call to undefined function

Hi All,

I'm new to CI (just started this week).

I am trying to get my own custom helper to work, but I keep getting 'Call to undefined function' errors.

Here's my code:

app/Helpers/build_page.php

PHP Code:

<?php

function tester(){
    return "test";
} 


app/Controllers/News.php (snippet)

PHP Code:

public function index()
    {
        helper('build_page');
        echo tester(); 


As read in the manual I guess this should work?
But I keep getting:

Call to undefined function App\Controllers\tester()

What am I missing here.