Case-sensitive grouping by tags field fails

Hello forum,

I tried to group a page collection by a tags field. All pages have a maximum of one tag stored in the tags field. It works when I use it with case-sensitivity turned off (default behavior):

$categories = page('bikes')
  ->children()
  ->filter('bikeBrand', $page->title())
  ->group('bikeCategory')

But it fails when I turn case-sensitivity on:

$categories = page('bikes')
  ->children()
  ->filter('bikeBrand', $page->title())
  ->group('bikeCategory', false)

Kirby shows the following error

Illegal offset type in isset or empty

happening here:

# kirby/src/Cms/Collection.php

if (isset($groups->data[$value]) === false) {

Does anyone have an idea why?