Problem with cyrilic encription in URL

Hello there, I have stupid question ...
I cant open post when the frendly URL is in Cyrilic 
For exaple : https://ciblog.marsislav.net/posts/тест

My code is:

<p><a class="btn btnCool" href="<?php echo site_url('/posts/'.$post['slug']); ?>">Read More</a></p>

Thank you!

$config['permitted_uri_chars'] = 'a-z 0-9~%.:_\-@';

\application\config\config.php :   $config['permitted_uri_chars'] = 'a-z 0-9~%.:_\-@ а-я';
and  $config['permitted_uri_chars'] = 'a-z 0-9~%.:_\-@'


Both did not work

It`blow up this message:

A PHP Error was encountered
Severity: Warning

Message: Trying to access array offset on value of type null

Filename: controllers/Posts.php

Line Number: 25

Backtrace:

File: C:\xampp\htdocs\ciblog\application\controllers\Posts.php
Line: 25
Function: _error_handler

File: C:\xampp\htdocs\ciblog\index.php
Line: 315
Function: require_once

404 Page Not Found
The page you requested was not found.

The backtrace shows that the controller has been called.
So there are no problems with the URL.