diff --git a/VERSION b/VERSION
--- a/VERSION
+++ b/VERSION
@@ -1,3 +1,5 @@
+0.4.3 (2013.03.21):
+    - Data.ByteString.Lex.Integral: Corrected a segmentation fault in packDecimal.
 0.4.2 (2013.03.20):
     - Data.ByteString.Lex.Integral: Improved packDecimal.
 0.4.1 (2012.00.00):
diff --git a/bytestring-lexing.cabal b/bytestring-lexing.cabal
--- a/bytestring-lexing.cabal
+++ b/bytestring-lexing.cabal
@@ -8,7 +8,7 @@
 Build-Type:     Simple
 
 Name:           bytestring-lexing
-Version:        0.4.2
+Version:        0.4.3
 Stability:      provisional
 Homepage:       http://code.haskell.org/~wren/
 Author:         wren ng thornton, Don Stewart
diff --git a/src/Data/ByteString/Lex/Integral.hs b/src/Data/ByteString/Lex/Integral.hs
--- a/src/Data/ByteString/Lex/Integral.hs
+++ b/src/Data/ByteString/Lex/Integral.hs
@@ -1,6 +1,6 @@
 {-# OPTIONS_GHC -Wall -fwarn-tabs #-}
 ----------------------------------------------------------------
---                                                    2013.03.20
+--                                                    2013.03.21
 -- |
 -- Module      :  Data.ByteString.Lex.Integral
 -- Copyright   :  Copyright (c) 2010--2013 wren ng thornton
@@ -772,10 +772,14 @@
 numDecimalDigits :: (Integral a) => a -> Int
 {-# INLINE numDecimalDigits #-}
 numDecimalDigits n0
-    | n0 < 0    = error (_numDecimalDigits ++ _negativeNumber)
-    -- BUG: need to check n0 to be sure we won't overflow Word64
-    | otherwise = go 1 (fromIntegral n0 :: Word64)
+    | n0 < 0     = error (_numDecimalDigits ++ _negativeNumber)
+    -- Unfortunately this causes significant (1.2x) slowdown since
+    -- GHC can't see it will always fail for types other than Integer...
+    | n0 > limit = numDigits 10 (toInteger n0)
+    | otherwise  = go 1 (fromIntegral n0 :: Word64)
     where
+    limit = fromIntegral (maxBound :: Word64)
+    
     fin n bound = if n >= bound then 1 else 0
     go k n
         | k `seq` False = undefined -- For strictness analysis