elm Task or port bug?

bug.elm

import Http
import Markdown
import Html exposing (Html)
import Task exposing (Task, andThen)
import Time exposing (every, second)


main : Signal Html
main = Signal.map Markdown.toHtml readme.signal

-- set up mailbox
--   the signal is piped directly to main
--   the address lets us update the signal
readme : Signal.Mailbox String
readme = Signal.mailbox "# try"

-- send some markdown to our readme mailbox
report : String -> Task x ()
report markdown = Signal.send readme.address markdown

-- get the readme *and then* send the result to our mailbox
port fetchReadme : Signal (Task Http.Error ())
port fetchReadme = Signal.map (\_ -> Http.getString readmeUrl `andThen` report) (every (30 * second))


-- the URL of the README.md that we desire
readmeUrl : String
readmeUrl = "http://package.elm-lang.org/packages/elm-lang/core/latest/README.md"

I get the following error at runtime:

undefined is not an object (evaluating 'node.kids.forEach')
Open the developer console for more details.

But I find nothing interesting in the console.

It appears it only occurs in elm-reactor in debug mode.