Plugin Help FactionsUUID ./f invite not working

My players are able to do all faction commands but for some reason they can't ./f invite everytime they ./f invite it says "An Internal error occured while attempting to perform this command" I've tried everything (The faction plugin I'm running is factionUUID), I'm currently using 1.8 spigot and I've tried everything and even searched online for help ad there was nothing, Hopefully i get the help i need.
Here is my console log:
http://pastebin.com/j0TvVnwf