diff --git a/Crypto/Random/Types.hs b/Crypto/Random/Types.hs
index da547b0..8899559 100644
--- a/Crypto/Random/Types.hs
+++ b/Crypto/Random/Types.hs
@@ -16,7 +16,7 @@ import Control.Applicative
 import Crypto.Random.Entropy
 import Crypto.Internal.ByteArray
 
-class Monad m => MonadRandom m where
+class (Functor m, Monad m) => MonadRandom m where
     getRandomBytes :: ByteArray byteArray => Int -> m byteArray
 
 class DRG gen where