diff --git a/Crypto.cabal b/Crypto.cabal
--- a/Crypto.cabal
+++ b/Crypto.cabal
@@ -1,5 +1,5 @@
 Name:            Crypto
-Version:         4.2.4
+Version:         4.2.5
 License:         OtherLicense
 License-File:    ReadMe.tex
 Author:          Dominic Steinitz
diff --git a/Data/Digest/SHA2.hs b/Data/Digest/SHA2.hs
--- a/Data/Digest/SHA2.hs
+++ b/Data/Digest/SHA2.hs
@@ -106,7 +106,7 @@
 data Hash384 = Hash384 !Word64 !Word64 !Word64 !Word64 !Word64 !Word64 deriving (Eq, Ord)
 data Hash224 = Hash224 !Word32 !Word32 !Word32 !Word32 !Word32 !Word32 !Word32 deriving (Eq, Ord)
 
-instance (Integral a) => Show (Hash8 a) where
+instance (Integral a, Show a) => Show (Hash8 a) where
  showsPrec _ (Hash8 a b c d e f g h) =
   (showHex a) . (' ':) .
   (showHex b) . (' ':) .
@@ -146,7 +146,7 @@
      where
       bs = bitSize (head r)
 
-instance (Integral h, Bits h) => Hash (Hash8 h) where
+instance (Integral h, Bits h, Show h) => Hash (Hash8 h) where
   toOctets (Hash8 x0 x1 x2 x3 x4 x5 x6 x7) = bitsToOctets =<< [x0, x1, x2, x3, x4, x5, x6, x7]
 
 instance Hash Hash384 where