diff --git a/Crypto/Random/Entropy.hs b/Crypto/Random/Entropy.hs
--- a/Crypto/Random/Entropy.hs
+++ b/Crypto/Random/Entropy.hs
@@ -90,7 +90,7 @@
 
 -- | Create a new entropy pool of a specific size
 --
--- While you can create as many entropy pool as you want, the pool can be shared between multiples RNGs.
+-- You can create as many entropy pools as you want, and a given pool can be shared between multiples RNGs.
 createEntropyPoolWith :: Int -> [EntropyBackend] -> IO EntropyPool
 createEntropyPoolWith poolSize backends = do
     when (null backends) $ fail "cannot get any source of entropy on this system"
diff --git a/Crypto/Random/Entropy/Windows.hs b/Crypto/Random/Entropy/Windows.hs
--- a/Crypto/Random/Entropy/Windows.hs
+++ b/Crypto/Random/Entropy/Windows.hs
@@ -8,31 +8,23 @@
 -- code originally from the entropy package and thus is:
 --   Copyright (c) Thomas DuBuisson.
 --
-{-# LANGUAGE ForeignFunctionInterface #-}
+{-# LANGUAGE CPP, ForeignFunctionInterface #-}
 module Crypto.Random.Entropy.Windows
     ( WinCryptoAPI
     ) where
 
 import Data.Int (Int32)
-import Data.Word (Word32, Word8)
+import Data.Word (Word8, Word32, Word64)
 import Foreign.C.String (CString, withCString)
 import Foreign.Ptr (Ptr, nullPtr)
 import Foreign.Marshal.Alloc (alloca)
 import Foreign.Marshal.Utils (toBool)
 import Foreign.Storable (peek)
+import System.Win32.Types (getLastError)
 
 import Crypto.Random.Entropy.Source
 
--- Define the constants we need from WinCrypt.h 
-msDefProv :: String
-msDefProv = "Microsoft Base Cryptographic Provider v1.0"
 
-provRSAFull :: Word32
-provRSAFull = 1
-
-cryptVerifyContext :: Word32
-cryptVerifyContext = 0xF0000000
-
 -- | handle to windows crypto API for random generation
 data WinCryptoAPI = WinCryptoAPI
 
@@ -43,26 +35,52 @@
     entropyGather WinCryptoAPI ptr n = do
         mctx <- cryptAcquireCtx
         case mctx of
-            Nothing  -> error "cannot re-grab win crypto api"
+            Nothing  -> do
+                lastError <- getLastError
+                fail $ "cannot re-grab win crypto api: error " ++ show lastError
             Just ctx -> do
                 r <- cryptGenRandom ctx ptr n
                 cryptReleaseCtx ctx
                 return r
     entropyClose WinCryptoAPI = return ()
 
+
+type DWORD = Word32
+type BOOL  = Int32
+type BYTE  = Word8
+
+#if defined(ARCH_X86)
+# define WINDOWS_CCONV stdcall
 type CryptCtx = Word32
+#elif defined(ARCH_X86_64)
+# define WINDOWS_CCONV ccall
+type CryptCtx = Word64
+#else
+# error Unknown mingw32 arch
+#endif
 
--- Declare the required CryptoAPI imports 
-foreign import stdcall unsafe "CryptAcquireContextA"
-   c_cryptAcquireCtx :: Ptr Word32 -> CString -> CString -> Word32 -> Word32 -> IO CryptCtx
-foreign import stdcall unsafe "CryptGenRandom"
-   c_cryptGenRandom :: CryptCtx -> Word32 -> Ptr Word8 -> IO Int32
-foreign import stdcall unsafe "CryptReleaseContext"
-   c_cryptReleaseCtx :: CryptCtx -> Word32 -> IO Int32
+-- Declare the required CryptoAPI imports
+foreign import WINDOWS_CCONV unsafe "CryptAcquireContextA"
+   c_cryptAcquireCtx :: Ptr CryptCtx -> CString -> CString -> DWORD -> DWORD -> IO BOOL
+foreign import WINDOWS_CCONV unsafe "CryptGenRandom"
+   c_cryptGenRandom :: CryptCtx -> DWORD -> Ptr BYTE -> IO BOOL
+foreign import WINDOWS_CCONV unsafe "CryptReleaseContext"
+   c_cryptReleaseCtx :: CryptCtx -> DWORD -> IO BOOL
 
+
+-- Define the constants we need from WinCrypt.h
+msDefProv :: String
+msDefProv = "Microsoft Base Cryptographic Provider v1.0"
+
+provRSAFull :: DWORD
+provRSAFull = 1
+
+cryptVerifyContext :: DWORD
+cryptVerifyContext = 0xF0000000
+
 cryptAcquireCtx :: IO (Maybe CryptCtx)
-cryptAcquireCtx = 
-    alloca $ \handlePtr -> 
+cryptAcquireCtx =
+    alloca $ \handlePtr ->
     withCString msDefProv $ \provName -> do
         r <- toBool `fmap` c_cryptAcquireCtx handlePtr nullPtr provName provRSAFull cryptVerifyContext
         if r
@@ -79,4 +97,6 @@
     success <- toBool `fmap` c_cryptReleaseCtx h 0
     if success
         then return ()
-        else fail "cryptReleaseCtx"
+        else do
+            lastError <- getLastError
+            fail $ "cryptReleaseCtx: error " ++ show lastError
diff --git a/crypto-random.cabal b/crypto-random.cabal
--- a/crypto-random.cabal
+++ b/crypto-random.cabal
@@ -1,5 +1,5 @@
 Name:                crypto-random
-Version:             0.0.8
+Version:             0.0.9
 Description:         Simple cryptographic random related types
 License:             BSD3
 License-file:        LICENSE
@@ -32,7 +32,12 @@
   else
     Build-Depends:  unix
     Other-modules:  Crypto.Random.Entropy.Unix
-    
+
+  if arch(i386)
+    cpp-options: -DARCH_X86
+  if arch(x86_64)
+    cpp-options: -DARCH_X86_64
+
   if arch(x86_64)
     cpp-options:    -DSUPPORT_RDRAND
     Other-modules:  Crypto.Random.Entropy.RDRand