Elm tests says that the module declaration is invalid, but why?

I'm very new to Elm and I'm starting my first pet project using Elm 0.19.

I want to create a test case the JSON decoding in my application. The JSON returned from the server looks like this (everything is defined in a file called Frontend.elm):

[
    {
        "gameId": "game1",
        "player1": "player1",
        "player2": "player2",
        "winner": "player1",
        "state": "ended"
    },
    {
        "gameId": "game3",
        "player1": "player1",
        "state": "joinable"
    }
]

My Elm models looks like this:

type Player =
  Player String

type State
  = Joinable
  | Started
  | Ended

type alias Game =
  { gameId : String
  , player1 : Maybe Player
  , player2 : Maybe Player
  , winner : Maybe Player
  , state : State}

and my decoding logic is defined like this:

gameStateDecoder : Decode.Decoder State
gameStateDecoder =
  string
        |> andThen (\stateAsString ->
           case stateAsString of
                "joinable" ->
                    succeed Joinable
                "ended" ->
                    succeed Ended
                "started" ->
                    succeed Started
                unknown ->
                    fail <| "Unknown game state: " ++ unknown
        )


playerDecoder : Decode.Decoder (Maybe Player)
playerDecoder =
    (maybe string)
        |> andThen (\maybePlayerString ->
            succeed
              <| case maybePlayerString of
                Just player ->
                   Just (Player player)
                _ ->
                   Nothing
        )

gameListDecoder : Decode.Decoder (List Game)
gameListDecoder =
  Decode.list gameDecoder

gameDecoder : Decode.Decoder Game
gameDecoder =
   Decode.map5 Game
        (field "gameId" string)
        (field "player1" playerDecoder)
        (field "player2" playerDecoder)
        (field "winner" playerDecoder)
        (field "state" gameStateDecoder)

Now I've tried to create a test case for this using the elm-explorations/test package:

import Frontend exposing (..)
import Expect exposing (equal)
import Test exposing (Test, test)
import Json.Decode exposing (decodeString)

decodesGameList : Test
decodesGameList =
    test "Properly decodes a game list" <|
        \() ->
            let
                json =
                    """
                      [
                          {
                              "gameId": "game",
                              "state": "joinable"
                          }
                      ]
                    """

                decodedOutput  =
                    decodeString gameListDecoder json
            in
                equal
                    decodedOutput
                    (Ok
                        [ Game "game" Nothing Nothing Nothing Joinable ]
                    )

but when I try to run it using elm-test I get this error:

<project path>/tests/FrontendTest.elm has an invalid module declaration. Check the first line of the file and make sure it has a valid module declaration there!

I don't understand what I'm doing wrong. The module is defined like this in Frontend.elm:

module Frontend exposing (gameListDecoder, State(..), Game, Player(..))

I can perfectly well compile it using:

$ elm make src/main/elm/Frontend.elm --output src/main/resources/static/index.html
Success! Compiled 1 module.

You can actually find the code at github.

How can I solve this?

An Elm module is expected to have a module declaration at the top of the file, i.e.

module FrontendTest exposing (..)

should fix the problem in this case.