diff --git a/src/Simplex/FileTransfer/Client/Main.hs b/src/Simplex/FileTransfer/Client/Main.hs
index 1eea6ef5a..079392a1b 100644
--- a/src/Simplex/FileTransfer/Client/Main.hs
+++ b/src/Simplex/FileTransfer/Client/Main.hs
@@ -44,7 +44,7 @@ import Data.List (foldl', sortOn)
 import Data.List.NonEmpty (NonEmpty (..), nonEmpty)
 import qualified Data.List.NonEmpty as L
 import Data.Map.Strict (Map)
-import qualified Data.Map as M
+import qualified Data.Map.Strict as M
 import Data.Maybe (fromMaybe, listToMaybe)
 import qualified Data.Text as T
 import Data.Word (Word32)
diff --git a/src/Simplex/FileTransfer/Description.hs b/src/Simplex/FileTransfer/Description.hs
index c702a177f..8c27d80e7 100644
--- a/src/Simplex/FileTransfer/Description.hs
+++ b/src/Simplex/FileTransfer/Description.hs
@@ -53,7 +53,7 @@ import Data.List (foldl', sortOn)
 import Data.List.NonEmpty (NonEmpty (..))
 import qualified Data.List.NonEmpty as L
 import Data.Map.Strict (Map)
-import qualified Data.Map as M
+import qualified Data.Map.Strict as M
 import Data.Maybe (fromMaybe)
 import Data.String
 import Data.Text (Text)
diff --git a/src/Simplex/FileTransfer/Server.hs b/src/Simplex/FileTransfer/Server.hs
index 819be9a81..be4c7d85f 100644
--- a/src/Simplex/FileTransfer/Server.hs
+++ b/src/Simplex/FileTransfer/Server.hs
@@ -475,7 +475,7 @@ processXFTPRequest HTTP2Body {bodyPart} = \case
                 pure FROk
               Left e -> do
                 us <- asks $ usedStorage . store
-                atomically . modifyTVar' us $ subtract (fromIntegral size)
+                atomically $ modifyTVar' us $ subtract (fromIntegral size)
                 liftIO $ whenM (doesFileExist fPath) (removeFile fPath) `catch` logFileError
                 pure $ FRErr e
           receiveChunk spec = do
@@ -571,7 +571,7 @@ withFileLog action = liftIO . mapM_ action =<< asks storeLog
 incFileStat :: (FileServerStats -> TVar Int) -> M ()
 incFileStat statSel = do
   stats <- asks serverStats
-  atomically $ modifyTVar (statSel stats) (+ 1)
+  atomically $ modifyTVar' (statSel stats) (+ 1)
 
 saveServerStats :: M ()
 saveServerStats =
diff --git a/src/Simplex/Messaging/Agent/Store/SQLite/Migrations.hs b/src/Simplex/Messaging/Agent/Store/SQLite/Migrations.hs
index 131561f4d..8e17d9af6 100644
--- a/src/Simplex/Messaging/Agent/Store/SQLite/Migrations.hs
+++ b/src/Simplex/Messaging/Agent/Store/SQLite/Migrations.hs
@@ -29,7 +29,7 @@ import Control.Monad (forM_, when)
 import qualified Data.Aeson.TH as J
 import Data.List (intercalate, sortOn)
 import Data.List.NonEmpty (NonEmpty)
-import qualified Data.Map as M
+import qualified Data.Map.Strict as M
 import Data.Maybe (isNothing, mapMaybe)
 import Data.Text (Text)
 import Data.Text.Encoding (decodeLatin1)
diff --git a/src/Simplex/Messaging/Agent/TRcvQueues.hs b/src/Simplex/Messaging/Agent/TRcvQueues.hs
index 3b02f64ae..38a60d6e0 100644
--- a/src/Simplex/Messaging/Agent/TRcvQueues.hs
+++ b/src/Simplex/Messaging/Agent/TRcvQueues.hs
@@ -63,7 +63,7 @@ addQueue rq (TRcvQueues qs cs) = do
     addQ = Just . maybe (k :| []) (k <|)
     k = qKey rq
 
--- Save time by aggregating modifyTVar
+-- Save time by aggregating modifyTVar'
 batchAddQueues :: (Foldable t, Queue q) => TRcvQueues q -> t q -> STM ()
 batchAddQueues (TRcvQueues qs cs) rqs = do
   modifyTVar' qs $ \now -> foldl' (\rqs' rq -> M.insert (qKey rq) rq rqs') now rqs
diff --git a/src/Simplex/Messaging/Server.hs b/src/Simplex/Messaging/Server.hs
index 87fe3e9cb..b583f578c 100644
--- a/src/Simplex/Messaging/Server.hs
+++ b/src/Simplex/Messaging/Server.hs
@@ -768,7 +768,7 @@ send th c@Client {sndQ, msgQ, sessionId} stats = do
       ts@((_, _, END) :| _) -> do -- END events are not combined with others
         let len = L.length ts
         atomically $ modifyTVar' (qSubEndSent stats) (+ len)
-        atomically $ modifyTVar' (qSubEndSentB stats) (+ len `div` 255) -- up to 255 ENDs in the batch
+        atomically $ modifyTVar' (qSubEndSentB stats) (+ (len `div` 255 + 1)) -- up to 255 ENDs in the batch
       _ -> pure ()
         
 sendMsg :: Transport c => MVar (THandleSMP c 'TServer) -> Client -> IO ()
@@ -1315,7 +1315,7 @@ client thParams' clnt@Client {subscriptions, ntfSubscriptions, rcvQ, sndQ, sessi
                   void $ setDelivered s msg
                 forkDeliver (rc@Client {sndQ = q}, s@Sub {delivered}, st) = do
                   t <- mkWeakThreadId =<< forkIO deliverThread
-                  atomically . modifyTVar' st $ \case
+                  atomically $ modifyTVar' st $ \case
                     -- this case is needed because deliverThread can exit before it
                     SubPending -> SubThread t
                     st' -> st'