UTF-8 warning during installation via `gem install`

[nodeg]

When installing tetra via gem I got the following warning:

$ gem install tetra
Fetching tetra-2.0.6.gem
Successfully installed tetra-2.0.6
Parsing documentation for tetra-2.0.6
unable to convert "\xEB" from ASCII-8BIT to UTF-8 for lib/template/bundled/apache-ant-1.9.7/KEYS, skipping
unable to convert "\xAE" from ASCII-8BIT to UTF-8 for lib/template/bundled/apache-ant-1.9.7/LICENSE, skipping
Installing ri documentation for tetra-2.0.6
Done installing documentation for tetra after 0 seconds
1 gem installed

Occurred on openSUSE Tumbleweed with the zsh shell and Ruby version 2.5.9 installed via rvm.

[nodeg]

I did some investigation and was not able to find a solution for fixing this, yet.