diff --git a/Yesod/Core/Internal/Run.hs b/Yesod/Core/Internal/Run.hs
--- a/Yesod/Core/Internal/Run.hs
+++ b/Yesod/Core/Internal/Run.hs
@@ -49,8 +49,20 @@
 import           Yesod.Core.Internal.Request  (parseWaiRequest,
                                                tooLargeResponse)
 import           Yesod.Routes.Class           (Route, renderRoute)
-import Control.DeepSeq (($!!))
+import Control.DeepSeq (($!!), NFData)
+import Control.Monad (liftM)
 
+returnDeepSessionMap :: Monad m => SessionMap -> m SessionMap
+#if MIN_VERSION_bytestring(0, 10, 0)
+returnDeepSessionMap sm = return $!! sm
+#else
+returnDeepSessionMap sm = fmap unWrappedBS `liftM` (return $!! fmap WrappedBS sm)
+
+-- | Work around missing NFData instance for bytestring 0.9.
+newtype WrappedBS = WrappedBS { unWrappedBS :: S8.ByteString }
+instance NFData WrappedBS
+#endif
+
 -- | Function used internally by Yesod in the process of converting a
 -- 'HandlerT' into an 'Application'. Should not be needed by users.
 runHandler :: ToTypedContent c
@@ -82,7 +94,7 @@
     state <- liftIO $ I.readIORef istate
 
     (finalSession, mcontents1) <- (do
-        finalSession <- return $!! ghsSession state
+        finalSession <- returnDeepSessionMap (ghsSession state)
         return (finalSession, Nothing)) `E.catch` \e -> return
             (Map.empty, Just $! HCError $! InternalError $! T.pack $! show (e :: E.SomeException))
 
diff --git a/Yesod/Core/Types.hs b/Yesod/Core/Types.hs
--- a/Yesod/Core/Types.hs
+++ b/Yesod/Core/Types.hs
@@ -318,8 +318,8 @@
 -- annotations to SetCookie in the cookie package.
 instance NFData Header where
     rnf (AddCookie x) = rnf x
-    rnf (DeleteCookie x y) = rnf x `seq` rnf y
-    rnf (Header x y) = rnf x `seq` rnf y
+    rnf (DeleteCookie x y) = x `seq` y `seq` ()
+    rnf (Header x y) = x `seq` y `seq` ()
 
 data Location url = Local url | Remote Text
     deriving (Show, Eq)
diff --git a/yesod-core.cabal b/yesod-core.cabal
--- a/yesod-core.cabal
+++ b/yesod-core.cabal
@@ -1,5 +1,5 @@
 name:            yesod-core
-version:         1.2.13
+version:         1.2.13.1
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>