diff --git a/Data/Primitive/ByteArray.hs b/Data/Primitive/ByteArray.hs
--- a/Data/Primitive/ByteArray.hs
+++ b/Data/Primitive/ByteArray.hs
@@ -212,7 +212,7 @@
                  -> Word8               -- ^ value to fill with
                  -> m ()
 {-# INLINE fillByteArray #-}
-fillByteArray (MutableByteArray dst#) doff c sz
+fillByteArray (MutableByteArray dst#) doff sz c
   = unsafePrimToPrim
   $ memset_mba dst# (fromIntegral doff) (fromIntegral c) (fromIntegral sz)
 
diff --git a/primitive.cabal b/primitive.cabal
--- a/primitive.cabal
+++ b/primitive.cabal
@@ -1,5 +1,5 @@
 Name:           primitive
-Version:        0.4
+Version:        0.4.0.1
 License:        BSD3
 License-File:   LICENSE
 Author:         Roman Leshchinskiy <rl@cse.unsw.edu.au>
@@ -12,6 +12,10 @@
         .
         This package provides wrappers for primitive array operations from
         GHC.Prim.
+        .
+        Changes in version 0.4.0.1
+        .
+        * Critical bug fix in @fillByteArray@
         .
         Changes in version 0.4
         .