diff --git a/monad-gen.cabal b/monad-gen.cabal
--- a/monad-gen.cabal
+++ b/monad-gen.cabal
@@ -1,5 +1,5 @@
 name:                monad-gen
-version:             0.2.0.2
+version:             0.2.0.3
 synopsis:            A simple monad for generating fresh integers
 description:
     This module provides a simple monad transformer @GenT@ to enumerate unique values within
diff --git a/src/Control/Monad/Gen.hs b/src/Control/Monad/Gen.hs
--- a/src/Control/Monad/Gen.hs
+++ b/src/Control/Monad/Gen.hs
@@ -67,7 +67,7 @@
 
 #if MIN_VERSION_mtl(2, 2, 1)
 #else
-instance MonadError e m => MonadError e (GenT e m) where
+instance MonadError e m => MonadError e (GenT e' m) where
   throwError = GenT . throwError
   catchError m h = GenT $ catchError (unGenT m) (unGenT . h)
 #endif
diff --git a/src/Control/Monad/Gen/Class.hs b/src/Control/Monad/Gen/Class.hs
--- a/src/Control/Monad/Gen/Class.hs
+++ b/src/Control/Monad/Gen/Class.hs
@@ -50,9 +50,9 @@
   gen = lift gen
 
 #if MIN_VERSION_mtl(2, 2, 1)
-instance (MonadGen e m) => MonadGen e (ExceptT e m) where
+instance (MonadGen e m) => MonadGen e (ExceptT e' m) where
   gen = lift gen
 #else
-instance (MonadGen e m, Error err) => MonadGen e (ErrorT err m) where
+instance (MonadGen e m, Error e') => MonadGen e (ErrorT e' m) where
   gen = lift gen
 #endif