diff --git a/OpenAFP.cabal b/OpenAFP.cabal
--- a/OpenAFP.cabal
+++ b/OpenAFP.cabal
@@ -1,10 +1,10 @@
 name:               OpenAFP
-version:            1.3
+version:            1.3.1
 license:            PublicDomain
 License-file:       LICENSE
 cabal-version:      >= 1.6
 author:             Audrey Tang
-Tested-With:        GHC==6.10.1, GHC==6.12.1, GHC==7.0.1, GHC==7.2.1
+Tested-With:        GHC==6.10.1, GHC==6.12.1, GHC==7.0.1, GHC==7.2.1, GHC==7.4.1
 maintainer:         audreyt@audreyt.org
 homepage:           https://github.com/audreyt/openafp/
 synopsis:           IBM AFP document format parser and generator
diff --git a/src/OpenAFP/Internals/Binary.hs b/src/OpenAFP/Internals/Binary.hs
--- a/src/OpenAFP/Internals/Binary.hs
+++ b/src/OpenAFP/Internals/Binary.hs
@@ -102,7 +102,7 @@
 instance Show A12 where show = zeropad 12
 
 {-# INLINE zeropad #-}
-zeropad :: (Integral a) => Int -> a -> String
+zeropad :: (Show a, Integral a) => Int -> a -> String
 zeropad len str = replicate len' '0' ++ str'
     where str' = map toUpper $ showHex str ""
           len' = 2*len - length str'