style_get_property giving wrong answer?

[Conscious User <conscioususer gmail com>]

I'm using gtk_widget_get_property to get the "toggle-spacing"
of a GtkMenuItem. However, it seems to be returning 0 even
though, visually, the spacing is clearly non-zero. What could
be causing this?

This is happening in Ubuntu 13.04 (GTK 3.6), and didn't happen
in previous versions. Does not seem to happen with other style
properties either.