diff --git a/System/IO/BlockIO.hs b/System/IO/BlockIO.hs
index d24c015..0c18f2e 100644
--- a/System/IO/BlockIO.hs
+++ b/System/IO/BlockIO.hs
@@ -3,7 +3,7 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ViewPatterns #-}
-
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE DerivingStrategies #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
@@ -24,6 +24,8 @@ module System.IO.BlockIO (
     IOResult(IOResult, IOError),
     ByteCount, Errno(..),
 
+    -- * Internal
+    pattern IOResult_
   ) where
 
 import Data.Word
@@ -64,7 +66,7 @@ data IOCtx = IOCtx {
 
                -- | Locking of the writer end of the URing: used by writers
                -- while they are modifying the uring submission queue.
-               ioctxURing :: !(MVar URing.URing),
+               ioctxURing :: !(MVar (Maybe URing.URing)),
 
                -- | Communication channel from writers to the completion thread:
                -- letting it know about new batches of IO that they have
@@ -98,7 +100,7 @@ initIOCtx IOCtxParams {ioctxBatchSizeLimit, ioctxConcurrencyLimit} = do
     mask_ $ do
       ioctxQSemN         <- newQSemN ioctxConcurrencyLimit
       uring              <- URing.setupURing (URing.URingParams ioctxBatchSizeLimit)
-      ioctxURing         <- newMVar uring
+      ioctxURing         <- newMVar (Just uring)
       ioctxChanIOBatch   <- newChan
       ioctxChanIOBatchIx <- newChan
       ioctxCloseSync     <- newEmptyMVar
@@ -131,14 +133,15 @@ initIOCtx IOCtxParams {ioctxBatchSizeLimit, ioctxConcurrencyLimit} = do
 
 closeIOCtx :: IOCtx -> IO ()
 closeIOCtx IOCtx {ioctxURing, ioctxCloseSync} = do
-    uring <- takeMVar ioctxURing
-    URing.prepareNop uring (URing.IOOpId maxBound)
-    URing.submitIO uring
-    takeMVar ioctxCloseSync
-    URing.closeURing uring
-    putMVar ioctxURing (throw closed)
-  where
-    closed = mkIOError ResourceVanished "IOCtx closed" Nothing Nothing
+    uringMay <- takeMVar ioctxURing
+    case uringMay of
+      Nothing -> putMVar ioctxURing Nothing
+      Just uring -> do
+        URing.prepareNop uring (URing.IOOpId maxBound)
+        URing.submitIO uring
+        takeMVar ioctxCloseSync
+        URing.closeURing uring
+        putMVar ioctxURing Nothing
 
 data IOOp = IOOpRead  !Fd !FileOffset !(Ptr Word8) !ByteCount
           | IOOpWrite !Fd !FileOffset !(Ptr Word8) !ByteCount
@@ -224,21 +227,24 @@ submitIO IOCtx {
                 iobatchOpCount,
                 iobatchCompletion
               }
-    withMVar ioctxURing $ \uring -> do
+    withMVar ioctxURing $ \case
+      Nothing -> throwIO closed
+      Just uring -> do
 --      print ("submitIO", iobatchOpCount)
-      sequence_
-        [ --print ioop >>
-          case ioop of
-           IOOpRead  fd off buf cnt ->
-             URing.prepareRead  uring fd off buf cnt ioopid
-
-           IOOpWrite fd off buf cnt ->
-             URing.prepareWrite uring fd off buf cnt ioopid
-        | (ioop, ioopix) <- zip ioops [IOOpIx 0 ..]
-        , let !ioopid = packIOOpId iobatchIx ioopix ]
-      URing.submitIO uring
+        sequence_
+          [ --print ioop >>
+            case ioop of
+            IOOpRead  fd off buf cnt ->
+              URing.prepareRead  uring fd off buf cnt ioopid
+
+            IOOpWrite fd off buf cnt ->
+              URing.prepareWrite uring fd off buf cnt ioopid
+          | (ioop, ioopix) <- zip ioops [IOOpIx 0 ..]
+          , let !ioopid = packIOOpId iobatchIx ioopix ]
+        URing.submitIO uring
 --      print ("submitIO", "submitting done")
     map (IOResult_ . coerce) . elems <$> takeMVar iobatchCompletion
+  where closed = mkIOError ResourceVanished "IOCtx closed" Nothing Nothing
 
 data IOBatch = IOBatch {
                  iobatchIx         :: !IOBatchIx,