Runtime error when using pushUrl with --debug flag on.

[ChristophP]

Compiling with the --debug flag makes this SSCCE crash with a runtime error when the button is clicked: key.a is not a function. There is no runtime error when --debug is ommitted

module Main exposing (..)

import Browser
import Browser.Events
import Browser.Navigation as Nav
import Html exposing (..)
import Html.Events exposing (onClick)
import Json.Decode as JD
import Tuple exposing (pair)
import Url
import Url.Builder

main =
    Browser.application
        { init = init
        , view = view
        , update = update
        , subscriptions = \_ -> Sub.none
        , onUrlRequest = LinkClicked
        , onUrlChange = UrlChange
        }

type alias Model =
    { key : Nav.Key }

init : () -> Url.Url -> Nav.Key -> ( Model, Cmd Msg )

init env _ key =
    pair { key = key } Cmd.none

view : Model -> Browser.Document Msg

view model =
    { title = "My app"
    , body =
        [ Html.h2 [] [ text "Elm 19" ]
        , button [ onClick ButtonClick ]
            [ text "Click Me" ]
        ]
    }

type Msg = LinkClicked Browser.UrlRequest | UrlChange Url.Url | ButtonClick

update msg model =
    case msg of
        LinkClicked urlRequest ->
            case urlRequest of
                Browser.Internal url ->
                    ( model, Nav.pushUrl model.key (Url.toString url))
                Browser.External url ->
                    ( model, Nav.load url)
        UrlChange url ->
            pair model Cmd.none
        ButtonClick ->
            pair model <| Nav.pushUrl model.key (Url.Builder.absolute [ "hello" ] [])