Need Help Creating Registration/Login Form

Hi guys! Total CI noob here! Just so we're on the same page, I'm following this tutorial: http://www.kodingmadesimple.com/2015/08/...ation.html

Which covers creating a registration form in Code Igniter with some validation and whatnot. My code looks exactly as it is shown in the tutorial.

I have my database set up:

   

My browser is displaying the form fine: 

   

But after filling out the fields and submitting the form, I get this error in Safari:

   

Any idea what's causing this? If you need any more information, I will be happy to give it to you.