haskell extensions question

hi, I am using the haskell extensions (in Hugs - Nov 2002) and I have some troubles with that. For example: class A a where a :: a -> String a = const "a" class B b where b :: b -> String b = const "b" instance (A b) => B b where b = a instance B () ----------------- In this case, Hugs says: ERROR "Test.hs":12 - Overlapping instances for class "B" *** This instance : B () *** Overlaps with : B a *** Common instance : B () but I don't have an instance "A ()" from which haskell can infer an instance "B ()". can somebody explain me that it is happening here? Thanks in advanced... Diego