PreviewWidget.add_attribute_value() doesn't accept array of objects

I'm trying to use a PreviewWidget of type audio but have had no success so far.

The C++ API docs say that the 'tracks' attribute expects a composite attribute containing an array of tuples and that it can be created by using a VariantBuilder. However, for javascript, the VariantBuilder class does not seem to be present.

I have tried to use an array of objects, like this:

var audio = new scopes.lib.PreviewWidget("audio", "audio");
audio.add_attribute_value(
    "tracks",
    [
        {
            "title": new scopes.lib.Variant(result.get("title")),
            "source": new scopes.lib.Variant(result.get("audio_preview_url"))
        }
    ]
);

This fails with the following error:

file:///usr/share/unity8//Dash/Previews/PreviewAudioPlayback.qml:145:35: Unable to assign [undefined] to QString

The PreviewWidget does get added and it includes a single track. The track does not show the title, and then when I try to click on the widget in order to hear the sound, I get this error:

file:///usr/share/unity8//Dash/Previews/PreviewAudioPlayback.qml:84: Error: Cannot assign [undefined] to QUrl

I've tried searching online for examples of this type, but came up short. Thanks for your help.