Nested types produce incorrect decoders

[arbus]

given the following Haskell types

data Foo =
    FooOne Text
  | FooTwo Bar
  deriving (Generic)
  deriving (Elm, ToJSON, FromJSON) via ElmStreet Foo

data Bar = Bar
  { barOne :: Int
  , barTwo :: Bool
  }
  deriving (Generic)
  deriving (Elm, ToJSON, FromJSON) via ElmStreet Bar

it encodes and decodes like so:

λ> let x = FooOne "Hello"
λ> encode x
"{\"tag\":\"FooOne\",\"contents\":\"Hello\"}"

λ> let y = FooTwo $ Bar 1 True
λ> encode y
"{\"tag\":\"FooTwo\",\"contents\":{\"two\":true,\"one\":1}}"

The generated elm code for Foo and Bar have the following decoder:

decodeFoo : Decoder Foo
decodeFoo =
    let decide : String -> Decoder Foo
        decide x = case x of
            "FooOne" -> D.field "contents" <| D.map FooOne (D.index 0 D.string)
            "FooTwo" -> D.field "contents" <| D.map FooTwo (D.index 0 decodeBar)
            c -> D.fail <| "Foo doesn't have such constructor: " ++ c
    in D.andThen decide (D.field "tag" D.string)

decodeBar : Decoder Bar
decodeBar = D.succeed Bar
    |> required "one" D.int
    |> required "two" D.bool

The problem is in decide function where it assumes that the values of the contents field will be an array. The generated decoders fail as can be seen here:

> D.decodeString ED.decodeFoo "{\"tag\":\"FooOne\",\"contents\":\"Hello\"}"
Err (Field "contents" (Failure ("Expecting an ARRAY") <internals>))
    : Result D.Error ElmStreet.Types.Foo
> D.decodeString ED.decodeFoo "{\"tag\":\"FooTwo\",\"contents\":{\"two\":true,\"one\":1}}"
Err (Field "contents" (Failure ("Expecting an ARRAY") <internals>))
    : Result D.Error ElmStreet.Types.Foo

[chshersh]

I'm looking into this.