After loading (cemerick.austin.repls/exec), austin fails with "No value supplied for key:"

Found the magic to get austin, piggieback and cljs to compile, but when running (cemeric.austin.repls/exec) it fails with the following error in the repl.

Please let me know what I can do to help track this down.

```
IllegalArgumentException No value supplied for key: cemerick.austin.DelegatingExecEnv@4fd7d1f4  clojure.lang.PersistentHashMap.create (PersistentHashMap.java:77)
```
