Can't decode session from elm port

Trying to get elm ports working to maintain the session.

In index.html, the script includes the following listener:

  window.addEventListener("load", function(event) {
    app.ports.onSessionChange.send(localStorage.session);
  }, false);

localStorage.session looks like this (and it stays there until I've logged out):

{"email":"user@fake.com","token":"eyJhbG...","user_id":1,"handle":"me"}

The definition in Ports.elm is:

port onSessionChange : (Value -> msg) -> Sub msg

This port is connected to Main.elm here (let me know if I've forgotten to include some of the definitions below):

subscriptions : Model -> Sub Msg
subscriptions model =
    Ports.onSessionChange sessionChange


sessionChange : Json.Decode.Value -> Msg
sessionChange value =
    let
        result =
            Json.Decode.decodeValue sessionDecoder value
    in
        case result of
            Ok sess ->
                SetSession (Just sess)

            Err err ->
                SetSession Nothing

    ...


type alias Session =
    { email : String
    , token : String
    , user_id : Int
    , handle : String
    }

    ...

import Json.Decode as Decode exposing (..)
import Json.Decode.Pipeline as Pipeline exposing (decode, required)

sessionDecoder : Decode.Decoder Session
sessionDecoder =
    Pipeline.decode Session
        |> Pipeline.required "email" Decode.string
        |> Pipeline.required "token" Decode.string
        |> Pipeline.required "user_id" Decode.int
        |> Pipeline.required "handle" Decode.string

    ...

type Msg
    = NoOp
    | SetSession (Maybe Session)

    ...


update msg model =
    case msg of
        SetSession session ->
            case Debug.log "session = " session of
                Just sess ->
                    ({ model | session = sess } , Cmd.none)

                Nothing ->
                    (model, Cmd.none)

Debug.log "session" displays Nothing in the console when the page loads, so JS is talking to elm, but the decoder seems to be failing. Any ideas?

I've plugged your code into a minimal working example and everything works fine. You might want to log the value of localStorage.session from inside the javascript portion to make sure it's a valid JSON value.