Thinking Sphinx won't build index

Question

I am currently trying to implement a search with Thinking-Sphinx (5.3.0) and Rails (6.1.4.2) running on Ruby 2.7.0 inside an Ubuntu 20.04.

When I run 'rake ts:rebuild' to build the index, it aborts with the message

"NameError: undefined method extend_object' for class Singleton' Did you mean? extended"

I have all prerequisites (gems 'mysql2' and 'thinking-sphinx', and Sphinx itself (2.2.11)) correctly installed, following the setup instructions at https://freelancing-gods.com/thinking-sphinx/v5/quickstart.html.

Does anybody have any idea?

Thanks in advance!