CSS text-shadow property not working with GtkTextView

Question

Does anyone of you know, why the CSS property text-shadow does not work with GtkTextview? It works pretty well with other widgets. (I tested other CSS properties and they work fine)

Edit: Minimal code snippet demonstrating the problem (in Vala, compile with valac test.vala --pkg gtk+-3.0)

using Gtk;

void load_css(Gtk.Window window)
{
    var css_provider = new Gtk.CssProvider();
    css_provider.load_from_data("textview * { text-shadow: 0 0 5px #000; color: #0f0; }");

    var screen = window.get_screen();
    Gtk.StyleContext.add_provider_for_screen(screen, css_provider, Gtk.STYLE_PROVIDER_PRIORITY_USER);
}

int main (string[] args)
{
    Gtk.init (ref args);

    var window = new Window();
    window.set_default_size(400, 400);
    window.destroy.connect(Gtk.main_quit);

    load_css(window);

    var textview = new Gtk.TextView();

    window.add(textview);
    window.show_all();

    Gtk.main();

    return 0;
}