diff --git a/checkers.cabal b/checkers.cabal
--- a/checkers.cabal
+++ b/checkers.cabal
@@ -1,5 +1,5 @@
 Name:                checkers
-Version:             0.4.9.2
+Version:             0.4.9.3
 Cabal-Version:       >= 1.6
 Synopsis:            Check properties on standard classes and data structures.
 Category:            Testing
diff --git a/src/Test/QuickCheck/Classes.hs b/src/Test/QuickCheck/Classes.hs
--- a/src/Test/QuickCheck/Classes.hs
+++ b/src/Test/QuickCheck/Classes.hs
@@ -134,7 +134,8 @@
   ( Functor m
   , Monoid (m a)
   , Monoid (m b)
-  , Arbitrary (a->b)
+  , CoArbitrary a
+  , Arbitrary b
   , Arbitrary (m a)
   , Show (m a)
   , EqProp (m b)) =>