cached-scripts page fatal error

[Steve8291]

I have just installed OpCacheGUI. It's fantastic! However, I keep getting a fatal error when I try to access the cached scripts page. It will show the cached scripts and allow me to Invalidate them but I lose the menu that allows me to navigate through "Status, Configuration, Cached Scripts, Graphs". Hitting the browser back button does bring me back. So it doesn't affect functionality but it is filling up my apache error.log
Here is the error:

Fatal error: Uncaught Error: Call to undefined function OpCacheGUI\Format\mb_substr() in /var/www/OpCacheGUI/src/OpCacheGUI/Format/Prefix.php:90 Stack trace: #0 /var/www/OpCacheGUI/src/OpCacheGUI/Format/Prefix.php(63): OpCacheGUI\Format\Prefix->findLongestPrefix('/var/www/OpCach...', '/var/www/OpCach...') #1 /var/www/OpCacheGUI/src/OpCacheGUI/Format/Prefix.php(34): OpCacheGUI\Format\Prefix->getPrefixLength(Array) #2 /var/www/OpCacheGUI/src/OpCacheGUI/OpCache/Status.php(286): OpCacheGUI\Format\Prefix->trim(Array) #3 /var/www/OpCacheGUI/template/cached.phtml(80): OpCacheGUI\OpCache\Status->getCachedScriptsForOverview(Object(OpCacheGUI\Format\Prefix)) #4 /var/www/OpCacheGUI/src/OpCacheGUI/Presentation/Html.php(79): require('/var/www/OpCach...') #5 /var/www/OpCacheGUI/src/OpCacheGUI/Presentation/Html.php(64): OpCacheGUI\Presentation\Html->renderTemplate('cached.phtml') #6 /var/www/OpCacheGUI/routes.php(51): OpCacheGUI\Presentation\Html->render('cached.phtml', Array) #7 /var/www/OpCacheGUI/src/OpCacheGUI/Network/Route.php(74): in /var/www/OpCacheGUI/src/OpCacheGUI/Format/Prefix.php on line 90