Test failed due to TypeError

I got the following error as a reason for my test (`zeroIsIdentityForAddition`) failing:

> This test failed because it threw an exception: "TypeError: Cannot read property '$' of undefined"

Here's the module under test (in particular look at the `plus` function):

```elm
module Natural.Unary exposing
( Natural
, zero
, isZero
, succ
, pred

, fromInt, toInt

, plus
)

type Natural = Natural (List ())

zero : Natural
zero =
Natural []

isZero : Natural -> Bool
isZero (Natural n) =
List.isEmpty n

succ : Natural -> Natural
succ (Natural n) =
Natural (() :: n)

pred : Natural -> Natural
pred (Natural n) =
Natural (Maybe.withDefault [] (List.tail n))

fromInt : Int -> Natural
fromInt n =
Natural (List.repeat n ())

toInt : Natural -> Int
toInt (Natural n) =
List.length n

plus : Natural -> Natural -> Natural
plus a b =
if isZero a then
succ b -- This is deliberate since I wanted to see how the test would fail.
-- But it didn't fail in the way I expected. Least of all due to a type error.
else
succ (plus (pred a) b)
```

And here's the failing test (in particular look at the `zeroIsIdentityForAddition` test):

```elm
module Test.Natural.Unary exposing
( zeroIsIdentityForAddition
)

import Expect
import Fuzz exposing (Fuzzer)
import Test exposing (Test, fuzz)

import Natural.Unary as Unary exposing (Natural)

zeroIsIdentityForAddition : Test
zeroIsIdentityForAddition =
fuzz naturalFuzzer "plus zero n == n for all natural n" <|
\n ->
Unary.plus Unary.zero n
|> Expect.equal n

-- HELPERS

naturalFuzzer : Fuzzer Natural
naturalFuzzer =
Fuzz.map Unary.fromInt (Fuzz.intRange 0 100)
```