diff --git a/HTk/Containers/Window.hs b/HTk/Containers/Window.hs
--- a/HTk/Containers/Window.hs
+++ b/HTk/Containers/Window.hs
@@ -109,7 +109,7 @@
   getGeometry win = cget win "geometry"
 
 -- | A window has a configureable size.
-instance Window w => HasSize w where
+instance (Window w, GUIObject w) => HasSize w where
   --  Sets the window\'s width.
   width w win = getGeometry win >>= \(_,h,x,y) -> geometry (w,h,x,y) win
   --  Gets the window\'s width.
@@ -130,7 +130,7 @@
   getSize win = getGeometry win >>= \(w,h,_,_) -> return (w,h)
 
 -- | A window has a position on the associated screen.
-instance Window w => HasPosition w where
+instance (Window w, GUIObject w) => HasPosition w where
   --  Sets the window\'s position-
   position (x,y) win =
     do
diff --git a/HTk/Menuitems/Menu.hs b/HTk/Menuitems/Menu.hs
--- a/HTk/Menuitems/Menu.hs
+++ b/HTk/Menuitems/Menu.hs
@@ -52,7 +52,7 @@
       cset w "menu" (show (objectname most))
 
 -- | Windows are containers for menus.
-instance Window w => HasMenu w
+instance (Window w, GUIObject w) => HasMenu w
 
 
 -- -----------------------------------------------------------------------
diff --git a/uni-htk.cabal b/uni-htk.cabal
--- a/uni-htk.cabal
+++ b/uni-htk.cabal
@@ -1,16 +1,16 @@
 name:           uni-htk
-version:        2.2.1.2
+version:        2.2.1.3
 build-type:     Simple
 license:        LGPL
 license-file:   LICENSE
 author:         uniform@informatik.uni-bremen.de
-maintainer:     c.maeder@jacobs-university.de
+maintainer:     chr.maeder@web.de
 homepage:       http://www.informatik.uni-bremen.de/htk/
 category:       GUI
 synopsis:       Graphical User Interface for Haskell Programs
 description:    GUI toolkit based on Tcl\/Tk
 cabal-version:  >= 1.4
-Tested-With:    GHC==6.10.4, GHC==6.12.3, GHC==7.0.4
+Tested-With:    GHC==8.0.1
 
 flag debug
   description: add debug traces