Call to undefined function untrailingslashit()

Hi. Our site has been throwing errors consistently with your plugin and we’re having trouble figuring out why. The complaint is a non-defined WP function, which I’m guessing is really there, but which the plugin is being fooled into thinking doesn’t exist:

Uncaught Error: Call to undefined function untrailingslashit() in /wp-content/plugins/social-share-buttons-by-supsystic/vendor/Rsc/Environment.php:102
Stack trace:
#0 /wp-content/plugins/social-share-buttons-by-supsystic/app/SupsysticSocialSharing.php(21): Rsc_Environment->__construct('sss', '2.1.4', '/srv/bindings/9...')
#1 /wp-content/plugins/social-share-buttons-by-supsystic/index.php(14): SupsysticSocialSharing->__construct()
#2 {main}
thrown

Any ideas?