diff --git a/Crypto/Number/Serialize.hs b/Crypto/Number/Serialize.hs
index 9533706..5ae3b9a 100644
--- a/Crypto/Number/Serialize.hs
+++ b/Crypto/Number/Serialize.hs
@@ -14,26 +14,23 @@ module Crypto.Number.Serialize
     , i2ospOf_
     ) where
 
-import           Data.Word
-import           Foreign.Storable
 import           Crypto.Number.Basic
 import           Crypto.Internal.Compat (unsafeDoIO)
-import qualified Crypto.Number.Serialize.Internal as Internal
 import qualified Crypto.Internal.ByteArray as B
+import qualified Crypto.Number.Serialize.Internal as Internal
 
 -- | os2ip converts a byte string into a positive integer
 os2ip :: B.ByteArrayAccess ba => ba -> Integer
-os2ip bs = unsafeDoIO $! B.withByteArray bs $ \src -> Internal.os2ip src (B.length bs)
+os2ip bs = unsafeDoIO $ B.withByteArray bs (\p -> Internal.os2ip p (B.length bs))
 
 -- | i2osp converts a positive integer into a byte string
 --
 -- first byte is MSB (most significant byte), last byte is the LSB (least significant byte)
 i2osp :: B.ByteArray ba => Integer -> ba
-i2osp 0 = B.unsafeCreate 1 $ \p -> pokeByteOff p 0 (0 :: Word8)
-i2osp m = B.unsafeCreate sz (\p -> Internal.i2osp m p sz >> return ())
+i2osp 0 = B.allocAndFreeze 1  (\p -> Internal.i2osp 0 p 1 >> return ())
+i2osp m = B.allocAndFreeze sz (\p -> Internal.i2osp m p sz >> return ())
   where
-    !sz = numBytes m
-{-# NOINLINE i2osp #-}
+        !sz = numBytes m
 
 -- | just like i2osp, but take an extra parameter for size.
 -- if the number is too big to fit in @len bytes, nothing is returned
@@ -42,10 +39,10 @@ i2ospOf :: B.ByteArray ba => Int -> Integer -> Maybe ba
 i2ospOf len m
     | len <= 0  = Nothing
     | m < 0     = Nothing
-    | otherwise = Just $ B.unsafeCreate len $ \p -> do
-        b <- Internal.i2ospOf m p len
-        if (b /= len) then error "invalid" else return ()
-{-# NOINLINE i2ospOf #-}
+    | sz > len  = Nothing
+    | otherwise = Just $ B.unsafeCreate len (\p -> Internal.i2ospOf m p len >> return ())
+  where
+        !sz = numBytes m
 
 -- | just like i2ospOf except that it doesn't expect a failure: i.e.
 -- an integer larger than the number of output bytes requested
@@ -54,4 +51,3 @@ i2ospOf len m
 -- the size (example the RSA modulo n).
 i2ospOf_ :: B.ByteArray ba => Int -> Integer -> ba
 i2ospOf_ len = maybe (error "i2ospOf_: integer is larger than expected") id . i2ospOf len
-