How to return JSON in Flask server built with Haxe

I'm trying to build a Flask server using Haxe, and I'm stumped on how to return a JSON. I got an example up and running using this gist that has Main.index() return a String, but when I try to return a String from haxe.Json.stringify() I get an error when I try to run the output Python.

import haxe.Constraints.Function;

@:pythonImport("flask", "Flask")
extern class Flask {
    function new(module:String);
    function route<T:Function>(path:String):T->T;
    function run():Void;
}

class Main {
    static function main() {
        var app = new Flask(untyped __name__);
        app.route("/")(index);
        app.run();
    }

    static function index() {
        return haxe.Json.stringify({msg:"hello"});
    }
}

Python error:

$ python main.py
  File "main.py", line 69
    return haxe_format_JsonPrinter.print(_hx_AnonObject({'msg': "hello"}),None,None)
                                       ^
SyntaxError: invalid syntax