diff --git a/json-rpc-generic.cabal b/json-rpc-generic.cabal
--- a/json-rpc-generic.cabal
+++ b/json-rpc-generic.cabal
@@ -1,5 +1,5 @@
 name:                json-rpc-generic
-version:             0.2.0.1
+version:             0.2.0.2
 synopsis:            Generic encoder and decode for JSON-RPC
 description:         This package contains generic encoder and decode for JSON-RPC
 homepage:            http://github.com/khibino/haskell-json-rpc-generic
diff --git a/test/Iso.hs b/test/Iso.hs
--- a/test/Iso.hs
+++ b/test/Iso.hs
@@ -7,9 +7,13 @@
 import qualified Data.Aeson as Aeson
 
 import Data.JsonRpc (Request, Response, Error, ErrorStatus, genericToArrayJSON)
+import qualified Data.JsonRpc.Failure as Error
 
 import Instances (Example)
 
+errorStatusCode :: ErrorStatus -> Bool
+errorStatusCode s =
+ Error.fromCode (Error.toCode s) == Just s
 
 aesonED :: (Eq a, ToJSON a, FromJSON a)
         => a -> Bool
@@ -22,8 +26,8 @@
 requestA r =
   Aeson.decode (Aeson.encode $ genericToArrayJSON <$> r) == Just r
 
-_errorStatus :: ErrorStatus -> Bool
-_errorStatus = aesonED
+errorStatus :: ErrorStatus -> Bool
+errorStatus = aesonED
 
 errorObj :: Error Example -> Bool
 errorObj = aesonED
@@ -33,8 +37,14 @@
 
 tests :: [Test]
 tests =
-  [ qcTest "iso - request" request
-  , qcTest "iso - request array" requestA
-  -- , qcTest "iso - error status" errorStatus
-  , qcTest "iso - error object" errorObj
-  , qcTest "iso - response" response ]
+  [ qcTest "iso - error status code"  errorStatusCode
+  ] ++
+
+  [ x
+  | x  <-  [ qcTest "iso JSON - error status" errorStatus ]
+  ,  Aeson.decode (Aeson.encode (1 :: Integer)) == Just (1 :: Integer)
+  ] ++
+  [ qcTest "iso JSON - request" request
+  , qcTest "iso JSON - request array" requestA
+  , qcTest "iso JSON - error object" errorObj
+  , qcTest "iso JSON - response" response ]