Fatal error: Call to undefined function mb_detect_encoding()

Fatal error: Call to undefined function mb_detect_encoding() in /wp-content/plugins/pro-sites/pro-sites-files/gateways/gateway-stripe-files/lib/Stripe/ApiRequestor.php on line 20

I have a multi-site set up with multi-db and WP 3.6.1. I want to use stripe as my gateway for Pro Sites. I have installed > deleted > installed again and still get the error.

When I go to the settings page and scroll down to the bottom and click save, it throws the error. When I put in my Stripe info, scroll down, and save, i get the error. I can not save any setting because of the error.

My only active Plugins are Google Analytics, Pro Sites and WPMU DEV Dashboard. Multi-DB is active, but it doesn't show up like the other plugins do.

How do i fix the error?