diff --git a/network-messagepack-rpc.cabal b/network-messagepack-rpc.cabal
--- a/network-messagepack-rpc.cabal
+++ b/network-messagepack-rpc.cabal
@@ -1,5 +1,5 @@
 name:                network-messagepack-rpc
-version:             0.1.1.2
+version:             0.1.1.4
 synopsis:            MessagePack RPC
 description:         [MessagePack RPC](https://github.com/msgpack-rpc/msgpack-rpc/blob/master/spec.md) library based on the "data-msgpack" package.
 homepage:            https://github.com/iij-ii/direct-hs/tree/master/network-messagepack-rpc
diff --git a/src/Network/MessagePack/RPC/Client.hs b/src/Network/MessagePack/RPC/Client.hs
--- a/src/Network/MessagePack/RPC/Client.hs
+++ b/src/Network/MessagePack/RPC/Client.hs
@@ -23,12 +23,16 @@
     )
 where
 
-import           Control.Concurrent      (forkFinally, forkIO, killThread)
+import           Control.Concurrent      (ThreadId, forkFinally, forkIO,
+                                          killThread)
+import           Control.Concurrent.MVar (MVar)
 import qualified Control.Concurrent.MVar as MVar
 import qualified Control.Exception.Safe  as E
 import           Control.Monad           (forever, void, when)
+import qualified Data.ByteString         as B
 import qualified Data.ByteString.Lazy    as BL
 import qualified Data.HashMap.Strict     as HM
+import           Data.IORef              (IORef)
 import qualified Data.IORef              as IORef
 import qualified Data.MessagePack        as MsgPack
 import           Data.Monoid             ((<>))
@@ -48,7 +52,7 @@
 
 data SessionState = SessionState {
     lastMessageId :: IORef MessageId
-  , dispatchTable :: IORef (HashMap MessageId (MVar Result))
+  , dispatchTable :: IORef (HM.HashMap MessageId (MVar Result))
   }
 
 -- | Result type of a RPC call.