Custom Sidebars Breaking Site

Hello,

I tried to update Custom Sidebars to the latest version and it keeps giving me the following fatal error:

Fatal error: Call to undefined function mb_strtolower() in /home/lifeforia/public_html/wp-content/plugins/custom-sidebars/inc/class-custom-sidebars.php on line 875

Any indication what the issue might be? Thanks in advance!