TYPO3 Core

Example:

    routes:
      - { routePath: '/filter/{semester}/{level}/{language}/{topic}', _controller: 'Something::index', _arguments: {'semester': 'semester', 'level': 'level', 'language': 'language', 'topic': 'topic'} }
    defaults:
      semester: '0'
      level: '0'
      language: '0'
      topic: ''

I want to have 'topic' as optional parameter.
The configuration above does not work.

`www.something.com/some/page/filter/1/2/3/something/` works
`www.something.com/some/page/filter/1/2/3/something` works
`www.something.com/some/page/filter/1/2/3//` does not work
`www.something.com/some/page/filter/1/2/3/` does not work
`www.something.com/some/page/filter/1/2/3` does not work

Solution for now is to add a second route without the topic