Flask jsonify: object is not iterable

I have a class looking like the following:

class Sound(SoundJsonSerializer):
    def __init__(self, name, length):
        self.name = name
        self.length = length

where SoundJsonSerializer enabling my custom JSONEncoder to serialize this object into JSON:

{
    "name": "foobar",
    "length": 23.4
}

Now I want one of my requests to respond exactly with the above JSON.

@app.route('/sounds/<soundid>')
def get_sound(soundid):
    s = Sound("foobar", 23.4)
    return jsonify(s)

yields an error claiming s was not iterable which is true. How do I make the method return my wanted JSON?

I know I can do it by explicitly creating a dict from my Sound object like this:

    return jsonify({"name": s.name, "length": s.length})

But this seems really ugly to me. What's the preferred way to achieve my goal?