diff --git a/Network/HTTP/Conduit/Manager.hs b/Network/HTTP/Conduit/Manager.hs
--- a/Network/HTTP/Conduit/Manager.hs
+++ b/Network/HTTP/Conduit/Manager.hs
@@ -21,6 +21,7 @@
     , failedConnectionException
     ) where
 
+import Debug.Trace
 #if !MIN_VERSION_base(4,6,0)
 import Prelude hiding (catch)
 #endif
@@ -65,7 +66,7 @@
 import Data.Default
 import Data.Maybe (mapMaybe)
 import System.IO (Handle)
-import System.Mem.Weak (addFinalizer)
+import System.Mem.Weak (Weak, deRefWeak)
 import Data.Conduit (($$), yield, runException)
 
 -- | Settings for a @Manager@. Please use the 'def' function and then modify
@@ -138,7 +139,7 @@
 takeSocket man key =
     I.atomicModifyIORef (mConns man) go
   where
-    go Nothing = (Nothing, Nothing)
+    go Nothing = trace "Using a closed manager" (Nothing, Nothing)
     go (Just m) =
         case Map.lookup key m of
             Nothing -> (Just m, Nothing)
@@ -184,8 +185,9 @@
                     return certStore
                 Just x -> return x
     mapRef <- I.newIORef (Just Map.empty)
+    wmapRef <- I.mkWeakIORef mapRef $ closeManager' mapRef
     certCache <- I.newIORef Map.empty
-    _ <- forkIO $ reap mapRef certCache
+    _ <- forkIO $ reap wmapRef certCache
     let manager = Manager
             { mConns = mapRef
             , mMaxConns = managerConnCount ms
@@ -193,18 +195,23 @@
             , mCertCache = certCache
             , mResponseTimeout = managerResponseTimeout ms
             }
-    addFinalizer manager $ closeManager manager
     return manager
 
 -- | Collect and destroy any stale connections.
-reap :: I.IORef (Maybe (Map.Map ConnKey (NonEmptyList ConnInfo)))
+reap :: Weak (I.IORef (Maybe (Map.Map ConnKey (NonEmptyList ConnInfo))))
      -> I.IORef (Map.Map S8.ByteString (Map.Map X509Encoded UTCTime))
      -> IO ()
-reap mapRef certCacheRef =
+reap wmapRef certCacheRef =
     mask_ loop
   where
     loop = do
         threadDelay (5 * 1000 * 1000)
+        mmapRef <- deRefWeak wmapRef
+        case mmapRef of
+            Nothing -> return () -- manager is closed
+            Just mapRef -> goMapRef mapRef
+
+    goMapRef mapRef = do
         now <- getCurrentTime
         let isNotStale time = 30 `addUTCTime` time >= now
         mtoDestroy <- I.atomicModifyIORef mapRef (findStaleWrap isNotStale)
@@ -313,8 +320,12 @@
 -- meaning you can safely use it without hurting any queries you may
 -- have concurrently running.
 closeManager :: Manager -> IO ()
-closeManager manager = mask_ $ do
-    m <- I.atomicModifyIORef (mConns manager) $ \x -> (Nothing, x)
+closeManager = closeManager' . mConns
+
+closeManager' :: I.IORef (Maybe (Map.Map ConnKey (NonEmptyList ConnInfo)))
+              -> IO ()
+closeManager' connsRef = mask_ $ do
+    m <- I.atomicModifyIORef connsRef $ \x -> (Nothing, x)
     mapM_ (nonEmptyMapM_ safeConnClose) $ maybe [] Map.elems m
 
 safeConnClose :: ConnInfo -> IO ()
diff --git a/http-conduit.cabal b/http-conduit.cabal
--- a/http-conduit.cabal
+++ b/http-conduit.cabal
@@ -1,5 +1,5 @@
 name:            http-conduit
-version:         1.9.4.2
+version:         1.9.4.3
 license:         BSD3
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>