sudo won't work but su does
I'm trying to install rails on OSX El Capitan 10.11.3, and it's trying to sudo. When I type in the root password it doesn't accept it (Sorry, try again). I know the password is correct because I can 'su' and it works, plus I can log in directly as root as a user and it works. I've disabled SIP. Any ideas?
MacBook Pro (Retina, 15-inch, Mid 2014)