diff --git a/serversession-backend-redis/src/Web/ServerSession/Backend/Redis/Internal.hs b/serversession-backend-redis/src/Web/ServerSession/Backend/Redis/Internal.hs
index 88f2a35..73c686a 100644
--- a/serversession-backend-redis/src/Web/ServerSession/Backend/Redis/Internal.hs
+++ b/serversession-backend-redis/src/Web/ServerSession/Backend/Redis/Internal.hs
@@ -18,6 +18,7 @@ module Web.ServerSession.Backend.Redis.Internal
   , getSessionImpl
   , deleteSessionImpl
   , removeSessionFromAuthId
+  , insertSessionForAuthId
   , deleteAllSessionsOfAuthIdImpl
   , insertSessionImpl
   , replaceSessionImpl
@@ -185,9 +186,19 @@ deleteSessionImpl sid = do
 -- | Remove the given 'SessionId' from the set of sessions of the
 -- given 'AuthId'.  Does not do anything if @Nothing@.
 removeSessionFromAuthId :: R.RedisCtx m f => SessionId -> Maybe AuthId -> m ()
-removeSessionFromAuthId _   Nothing       = return ()
-removeSessionFromAuthId sid (Just authId) =
-  void $ R.srem (rAuthKey authId) [rSessionKey sid]
+removeSessionFromAuthId = fooSessionBarAuthId R.srem
+
+-- | Insert the given 'SessionId' into the set of sessions of the
+-- given 'AuthId'.  Does not do anything if @Nothing@.
+insertSessionForAuthId :: R.RedisCtx m f => SessionId -> Maybe AuthId -> m ()
+insertSessionForAuthId = fooSessionBarAuthId R.sadd
+
+
+-- | (Internal) Helper for 'removeSessionFromAuthId' and 'insertSessionForAuthId'
+fooSessionBarAuthId
+  :: R.RedisCtx m f => (ByteString -> [ByteString] -> m (f Integer)) -> SessionId -> Maybe AuthId -> m ()
+fooSessionBarAuthId _   _   Nothing       = return ()
+fooSessionBarAuthId fun sid (Just authId) = void $ fun (rAuthKey authId) [rSessionKey sid]
 
 
 -- | Delete all sessions of the given auth ID.
@@ -200,21 +211,40 @@ deleteAllSessionsOfAuthIdImpl authId = do
 -- | Insert a new session.
 insertSessionImpl :: Session -> R.Redis ()
 insertSessionImpl session = do
-  transaction $ do
-    let sk = rSessionKey $ sessionKey session
-    r <- R.hmset sk (printSession session)
-    -- TODO: R.expireat
-    maybe (return ()) (\authId -> void $ R.sadd (rAuthKey authId) [sk]) $ sessionAuthId session
-    return (() <$ r)
+  -- Check that no old session exists.
+  let sid = sessionKey session
+  moldSession <- getSessionImpl sid
+  case moldSession of
+    Just oldSession -> liftIO $ E.throwIO $ SessionAlreadyExists oldSession session
+    Nothing -> do
+      transaction $ do
+        let sk = rSessionKey sid
+        r <- R.hmset sk (printSession session)
+        -- TODO: R.expireat
+        insertSessionForAuthId (sessionKey session) (sessionAuthId session)
+        return (() <$ r)
 
 
 -- | Replace the contents of a session.
 replaceSessionImpl :: Session -> R.Redis ()
 replaceSessionImpl session = do
-  -- Remove the old auth ID from the map if it has changed.
-  oldSession <- getSessionImpl (sessionKey session)
-  let oldAuthId = sessionAuthId =<< oldSession
-  when (oldAuthId /= sessionAuthId session) $
-    removeSessionFromAuthId (sessionKey session) oldAuthId
-  -- Otherwise the operation is the same as inserting.
-  insertSessionImpl session
+  -- Check that the old session exists.
+  let sid = sessionKey session
+  moldSession <- getSessionImpl sid
+  case moldSession of
+    Nothing -> liftIO $ E.throwIO $ SessionDoesNotExist session
+    Just oldSession -> do
+      transaction $ do
+        -- Delete the old session and set the new one.
+        let sk = rSessionKey sid
+        _ <- R.del [sk]
+        r <- R.hmset sk (printSession session)
+
+        -- Remove the old auth ID from the map if it has changed.
+        let oldAuthId = sessionAuthId oldSession
+            newAuthId = sessionAuthId session
+        when (oldAuthId /= newAuthId) $ do
+          removeSessionFromAuthId sid oldAuthId
+          insertSessionForAuthId sid newAuthId
+
+        return (() <$ r)
diff --git a/serversession-backend-redis/tests/Main.hs b/serversession-backend-redis/tests/Main.hs
index 4574d43..ffe74c5 100644
--- a/serversession-backend-redis/tests/Main.hs
+++ b/serversession-backend-redis/tests/Main.hs
@@ -5,8 +5,6 @@ import Test.Hspec
 import Web.ServerSession.Backend.Redis
 import Web.ServerSession.Core.StorageTests
 
-import qualified Control.Exception as E
-
 main :: IO ()
 main = do
   conn <- connect defaultConnectInfo