Open a dialog using openDialog and pass an object parameter

Question

I'm developing a Firefox extension which has this code to open some dialog:

Cu.import("resource://gre/modules/Services.jsm");

function showPanel()
{
    let mainWindow = Services.wm.getMostRecentWindow("navigator:browser");

    mainWindow.openDialog(data.url("ui/options.html"),
                          "Dictionary settings",
                          "chrome,centerscreen,dialog,modal,resizable,scrollbars,height=400,width=400",
                          { a: 123 });
}

On the dialog side, when I access window.arguments[0] I get an empty object with no fields. If I pass primitives like strings and numbers, they are passed correctly to the dialog.

What's the problem with this code?

Regards,

Update:

As I won't be able to commit my code, I've created a whole branch of the project. Please check this repository https://github.com/ashrafsabrym/firefox-dict-switcher

Using an object or array as parameter sends an empty object to the dialog, while primitives work well.