Custom Exceptions in Clojure?

I've been trying to create a user-defined exception in Clojure, and have been having all sorts of problems. I tried the method outlined here:

http://en.wikibooks.org/wiki/Clojure_Programming/Concepts#User-Defined_Exceptions

(gen-and-load-class 'user.MyException :extends Exception)

But that doesn't seem to work in Clojure 1.2 (or I'm doing something wrong...). My environment is Clojure 1.2, Emacs, and lein swank.

Thanks for your help!