[Vala] cannot create instance of abstract (non-instantiatable) type `GtkBox'

[Ronaldo Nascimento <sgtnasty gmail com>]

When I use Glade 3.12 to create a vertical box, it saves it as GtkBox.
However at runtime it crashes vala with:

(nwars:9024): GLib-GObject-WARNING **: cannot create instance of abstract
(non-instantiatable) type `GtkBox'

When I look at the XML the entry for the offending GtkBox is:
<object class="GtkBox" id="box1">

However if I re-do it in Glade 2 the entry for the VBox is:
<object class="GtkVBox" id="vbox1">

Is vala not ready for Gtk+3?

I used vala-gen-project to create this mess.
-- 
Ronaldo Nascimento  =][=