Plugin broke my installation

Tried to activate this plugin and immediately got a fatal error to an undefined function.

Fatal error: Call to undefined function Nette\Utils\mb_strtoupper() in /wp-content/plugins/simple-subscribe/libs/Nette/Nette.min.php on line 7191

If anyone reads this, you’ll need to remove the plugin via an ftp client otherwise wp-admin won’t be accessible. This is weird but I did install this plugin on my local installation of wordpress, and it worked just fine. Maybe some file got lost in the download process?

Spoke too soon. I uploaded my local installation of this plugin to my server and fatal error still triggers. But it works on a local install. WTH?!?!?!

https://www.ads-software.com/plugins/simple-subscribe/