Fatal Error

Hello I get the following error when adding the shortcode to a page:

Fatal error: Call to undefined function mb_detect_encoding() in /ebs/home/jpmonaganlaw/stage.jpmonaganlaw.com/wp-content/plugins/avvo-reviews-plugin-for-lawyers/simple_html_dom.php on line 1234

Here is my shortcode:
[reviews acc=”https://www.avvo.com/attorneys/12180-ny-joanne-monagan-895180.html” rating=”5″]