Running main.lua results in attempt to call method 'getAnimalId' (a nil value)

> $ lua main.lua

```
lua: main.lua:43: attempt to call method 'getAnimalId' (a nil value)
stack traceback:
    main.lua:43: in main chunk
    [C]: ?
```

In the end I gave up and tried http://lua-users.org/wiki/SimpleLuaClasses instead, that worked.
