Can't change shell in EC2 server

I am trying to change the default shell on Ubuntu server 13.10 on Amazon's AWS (AMI ami-ad184ac4).

I've tried:

~$ chsh -s /bin/zsh

But got:

Password: 
chsh: PAM: Authentication failure

I've tried obvious passwords (e.g. ubuntu) but always got the same error message - chsh: PAM: Authentication failure.

Any idea why can't I change the default shell?

Solution 1:

Oddly enough, using sudo with the username was the answer:

$ sudo chsh ubuntu -s /bin/zsh