Modal dialogs in Gtk

[WoodHacker]

Can anyone explain why my modal dialogs are not freezing the screen?
I can click on the window behind the dialog window and the dialog
disappears.

dialog = Gtk:ialog.new(Utils.getT("test"),
$main_application_window,
Gtk:ialog:ESTROY_WITH_PARENT)

dialog.width_request = 600
dialog.modal = true
dialog.window_position = Gtk::Window:OS_CENTER_ALWAYS
dialog.transient_for = $main_application_window

........

Everything else about it works fine.

Bill