Original Error

After enabling CKEditor “styles” for the default “content” text format, and clicking on save, I got the following error:

1

AssertionError: assert(NestedArray::keyExists($subform, $parts)) in assert() (line 869 of core/modules/ckeditor5/src/Plugin/Editor/CKEditor5.php).

(Temporary) Resolution

It is still an active issue that is pending to be solved: drupal-issue-3500153; But if you really need the “style” , you can temporary disable “Source editing”, and then you can save the “style” normally like you should (this is proposed by bjcooper on #28):

2026-07-18T084912