'TypeError: currentSubs[i] is not a function' when using ports in Elm 0.19 - elm

Question

I am attempting to send data from Elm 0.19 to JavaScript using ports.
Edit: The problem seems to be related to running/building with elm-app
In Elm, I declare an outgoing port:
port modelToJs : Json.Encode.Value -> Cmd msg
which I use in the update function to produce a Cmd that sends a JSON encoded value to JavaScript.
In JS, I instantiate the Elm app:
const app = Elm.Main.init({
node: document.getElementById('root')
});
and register the data handler:
app.ports.modelToJs.subscribe(function dataHandler(data) {
console.log("got from Elm:" + data);
});
When modelToJs is called, the data is not sent and printed to the console. Instead, I get the following JavasScript runtime error (which Elm claims to avoid by design):
TypeError: currentSubs[i] is not a function
var value = _Json_unwrap(converter(cmdList.a));
2160 | for (var i = 0; i < currentSubs.length; i++)
2161 | {
> 2162 | currentSubs[i](value);
2163 | }
2164 | }
2165 | return init;
I have also provided a full proof of concept project on GitHub: https://github.com/mpgirro/elm0.19-ports-issue
The repo also contains an image of the error message (sry, I lack the reputation to post images)

Answer 1

The error appears to be in dataHandler.js. It currently contains this:
function dataHandler(data) {
console.log("got from Elm:" + data);
}
If you declare the function as export default the problem goes away:
export default function dataHandler(data) {
console.log("got from Elm:" + data);
}