error compiling samples, greeter, casting to LibGtk::Widget*

[megatux]

crystal build samples/greeter.cr 
Error in samples/greeter.cr:27: instantiating 'greet(Gtk::Button+, Gtk::Window, Gtk::Label, Gtk::Entry)'

  greet button, info, info_text, name
  ^~~~~

in samples/greeter.cr:12: instantiating 'Gtk::Button+#sensitive=(Bool)'

  button.sensitive = false
         ^~~~~~~~~

in src/generated/gtk/widget.cr:1354: can't cast (Pointer(LibGtk::Button) | Pointer(LibGtk::CheckButton) | Pointer(LibGtk::ColorButton) | Pointer(LibGtk::FontButton) | Pointer(LibGtk::LinkButton) | Pointer(LibGtk::LockButton) | Pointer(LibGtk::MenuButton) | Pointer(LibGtk::ModelButton) | Pointer(LibGtk::RadioButton) | Pointer(LibGtk::ScaleButton) | Pointer(LibGtk::ToggleButton) | Pointer(LibGtk::VolumeButton)) to Pointer(LibGtk::Widget)

      LibGtk.widget_set_sensitive(to_unsafe.as(LibGtk::Widget*), sensitive)
                                  ^

libgtk3 version: 3.18.9-1ubuntu3.3
Crystal version 0.23.1 [e2a1389e8] (2017-07-13) LLVM 3.8.1