Creating a custom word list using system/user/config/captcha.php seems to have no effect

I created a file in system/user/config/captcha.php to overrule the standard captchas. It has the following content.

<?php

return [
  'accordion',
  'guitar',
  'bagpipes',
  'banjo',
  'drums',
  'bugle',
  'cello',
  'clarinet',
  'cymbals',
  'drums',
  'flute',
  'horn',
  'harmonica',
  'harp',
  'keyboard',
  'maracas',
  'organ',
  'piano',
  'recorder',
  'saxophone',
  'sitar',
  'tambourine',
  'triangle',
  'trombone',
  'trumpet',
  'tuba',
  'ukulele',
  'violin',
  'xylophone',
  'bassoon',
  'castanets',
  'didgeridoo',
  'bass',
  'gong',
  'harpsichord',
  'lute',
  'mandolin',
  'oboe',
  'piccolo',
  'viola'
];

?>

When I load the page that contains my form I can see it still uses the standard dictionary. Anybody knows where it goes wrong here?

Thanks!