diff --git a/Data/Attoparsec/Binary.hs b/Data/Attoparsec/Binary.hs
index fab76c6..01d6c8b 100644
--- a/Data/Attoparsec/Binary.hs
+++ b/Data/Attoparsec/Binary.hs
@@ -23,7 +23,7 @@ import Data.Word
 byteSize :: (Bits a) => a -> Int
 byteSize = (`div` 8) . bitSize
 
-pack :: (Bits a) => B.ByteString -> a
+pack :: (Bits a, Num a) => B.ByteString -> a
 pack = B.foldl' (\n h -> (n `shiftL` 8) .|. fromIntegral h) 0
 
 anyWordN :: (Bits a) => (B.ByteString -> a) -> Parser a
@@ -84,4 +84,4 @@ word64be = wordN unpack
 
 -- |Match a specific 64-bit little-endian word.
 word64le :: Word64 -> Parser Word64
-word64le = wordN $ B.reverse . unpack
\ No newline at end of file
+word64le = wordN $ B.reverse . unpack