diff --git a/Test/QuickCheck/Function.hs b/Test/QuickCheck/Function.hs
index a1affaf..7c730bd 100644
--- a/Test/QuickCheck/Function.hs
+++ b/Test/QuickCheck/Function.hs
@@ -206,6 +206,7 @@ shrinkFun shr (p :+: q) =
   [ p' .+. q  | p' <- shrinkFun shr p ] ++
   [ p  .+. q' | q' <- shrinkFun shr q ]
  where
+  isNil :: (a :-> b) -> Bool
   isNil Nil = True
   isNil _   = False