Validation min_length and max_length issue

Hi all!

\Config\Validation.php

PHP Code:

public $validateForm = [
        'name'        => 'required|min_length[4]|trim',
]; 


Controller:
$this->validation->run($this->request->getPost(), 'validateForm');

Error:
Call to undefined function CodeIgniter\Validation\mb_strlen()
BASEPATH/Validation/Rules.php at line 259


same error if use max_length