diff --git a/.travis.yml b/.travis.yml
index 1e67206..1d712c9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,12 +10,12 @@ dist: trusty
 language: c
 
 services:
- - redis-server
+  - redis-server
 
 addons:
   apt:
     packages:
-    - libgmp-dev
+      - libgmp-dev
   postgresql: "9.3"
 
 cache:
@@ -25,8 +25,7 @@ cache:
 
 matrix:
   include:
-    - env: STACKARGS="--resolver=lts-11"
-    - env: STACKARGS="--resolver=lts-12"
+    - env: STACKARGS="--resolver=lts-15"
     - env: STACKARGS="--resolver=nightly"
   allow_failures:
     - env: STACKARGS="--resolver=nightly"
diff --git a/examples/serversession-example-yesod-persistent/Model.hs b/examples/serversession-example-yesod-persistent/Model.hs
index cbe20c4..91ab7b5 100644
--- a/examples/serversession-example-yesod-persistent/Model.hs
+++ b/examples/serversession-example-yesod-persistent/Model.hs
@@ -1,3 +1,6 @@
+{-# LANGUAGE DerivingStrategies   #-}
+{-# LANGUAGE StandaloneDeriving   #-}
+{-# LANGUAGE UndecidableInstances #-}
 module Model where
 
 import ClassyPrelude.Yesod
diff --git a/serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid/Internal.hs b/serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid/Internal.hs
index 4b353db..3f06379 100644
--- a/serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid/Internal.hs
+++ b/serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid/Internal.hs
@@ -109,14 +109,14 @@ instance SafeCopy SS.SessionMap where
 
 -- | We can't @deriveSafeCopy 0 'base ''SS.SessionId@ as
 -- otherwise we'd require an unneeded @SafeCopy sess@.
-instance SafeCopy (SS.SessionId sess) where
+instance Typeable sess => SafeCopy (SS.SessionId sess) where
   putCopy = contain . safePut . SSI.unS
   getCopy = contain $ SSI.S <$> safeGet
 
 
 -- | We can't @deriveSafeCopy 0 'base ''SS.Session@ due to the
 -- required context.
-instance SafeCopy (SS.Decomposed sess) => SafeCopy (SS.Session sess) where
+instance (Typeable sess, SafeCopy (SS.Decomposed sess)) => SafeCopy (SS.Session sess) where
   putCopy (SS.Session key authId data_ createdAt accessedAt) = contain $ do
     put_t <- getSafePut
     safePut key
@@ -136,7 +136,7 @@ instance SafeCopy (SS.Decomposed sess) => SafeCopy (SS.Session sess) where
 
 -- | We can't @deriveSafeCopy 0 'base ''ServerSessionAcidState@ due
 -- to the required context.
-instance SafeCopy (SS.Decomposed sess) => SafeCopy (ServerSessionAcidState sess) where
+instance (Typeable sess, SafeCopy (SS.Decomposed sess)) => SafeCopy (ServerSessionAcidState sess) where
   putCopy (ServerSessionAcidState sits aits) = contain $ do
     safePut (HM.toList sits)
     safePut (HM.toList aits)
@@ -273,23 +273,23 @@ data DeleteAllSessionsOfAuthId sess = DeleteAllSessionsOfAuthId SS.AuthId derivi
 data InsertSession sess = InsertSession (SS.Session sess) deriving (Typeable)
 data ReplaceSession sess = ReplaceSession (SS.Session sess) deriving (Typeable)
 
-instance SafeCopy (GetSession sess) where
+instance Typeable sess => SafeCopy (GetSession sess) where
   putCopy (GetSession v) = contain $ safePut v
   getCopy = contain $ GetSession <$> safeGet
 
-instance SafeCopy (DeleteSession sess) where
+instance Typeable sess => SafeCopy (DeleteSession sess) where
   putCopy (DeleteSession v) = contain $ safePut v
   getCopy = contain $ DeleteSession <$> safeGet
 
-instance SafeCopy (DeleteAllSessionsOfAuthId sess) where
+instance Typeable sess => SafeCopy (DeleteAllSessionsOfAuthId sess) where
   putCopy (DeleteAllSessionsOfAuthId v) = contain $ safePut v
   getCopy = contain $ DeleteAllSessionsOfAuthId <$> safeGet
 
-instance SafeCopy (SS.Decomposed sess) => SafeCopy (InsertSession sess) where
+instance (Typeable sess, SafeCopy (SS.Decomposed sess)) => SafeCopy (InsertSession sess) where
   putCopy (InsertSession v) = contain $ safePut v
   getCopy = contain $ InsertSession <$> safeGet
 
-instance SafeCopy (SS.Decomposed sess) => SafeCopy (ReplaceSession sess) where
+instance (Typeable sess, SafeCopy (SS.Decomposed sess)) => SafeCopy (ReplaceSession sess) where
   putCopy (ReplaceSession v) = contain $ safePut v
   getCopy = contain $ ReplaceSession <$> safeGet
 
@@ -322,8 +322,8 @@ instance AcidContext sess => Method (ReplaceSession sess) where
 
 instance AcidContext sess => IsAcidic (ServerSessionAcidState sess) where
   acidEvents =
-    [ QueryEvent  $ \(GetSession sid)                   -> getSession sid
-    , UpdateEvent $ \(DeleteSession sid)                -> deleteSession sid
-    , UpdateEvent $ \(DeleteAllSessionsOfAuthId authId) -> deleteAllSessionsOfAuthId authId
-    , UpdateEvent $ \(InsertSession session)            -> insertSession session
-    , UpdateEvent $ \(ReplaceSession session)           -> replaceSession session ]
+    [ QueryEvent  (\(GetSession sid)                   -> getSession sid) safeCopyMethodSerialiser
+    , UpdateEvent (\(DeleteSession sid)                -> deleteSession sid) safeCopyMethodSerialiser
+    , UpdateEvent (\(DeleteAllSessionsOfAuthId authId) -> deleteAllSessionsOfAuthId authId) safeCopyMethodSerialiser
+    , UpdateEvent (\(InsertSession session)            -> insertSession session) safeCopyMethodSerialiser
+    , UpdateEvent (\(ReplaceSession session)           -> replaceSession session) safeCopyMethodSerialiser ]
diff --git a/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs b/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs
index 623857c..36cc39e 100644
--- a/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs
+++ b/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs
@@ -96,6 +96,7 @@ instance forall sess. P.PersistFieldSql (Decomposed sess) => P.PersistEntity (Pe
         ["Eq", "Ord", "Show", "Typeable"]
         M.mempty
         False
+        Nothing
     where
       pfd :: P.EntityField (PersistentSession sess) typ -> P.FieldDef
       pfd = P.persistFieldDef
@@ -142,8 +143,10 @@ instance forall sess. P.PersistFieldSql (Decomposed sess) => P.PersistEntity (Pe
                  (P.SqlOther "SqlType unset for key")
                  []
                  True
-                 P.NoReference]
+                 P.NoReference
+                 Nothing]
               []))
+        Nothing
   persistFieldDef PersistentSessionKey
     = P.FieldDef
         (P.HaskellName "key")
@@ -153,6 +156,7 @@ instance forall sess. P.PersistFieldSql (Decomposed sess) => P.PersistEntity (Pe
         ["maxlen=30"]
         True
         P.NoReference
+        Nothing
   persistFieldDef PersistentSessionAuthId
     = P.FieldDef
         (P.HaskellName "authId")
@@ -162,6 +166,7 @@ instance forall sess. P.PersistFieldSql (Decomposed sess) => P.PersistEntity (Pe
         ["Maybe", "default=NULL"]
         True
         P.NoReference
+        Nothing
   persistFieldDef PersistentSessionSession
     = P.FieldDef
         (P.HaskellName "session")
@@ -171,6 +176,7 @@ instance forall sess. P.PersistFieldSql (Decomposed sess) => P.PersistEntity (Pe
         []
         True
         P.NoReference
+        Nothing
   persistFieldDef PersistentSessionCreatedAt
     = P.FieldDef
         (P.HaskellName "createdAt")
@@ -180,6 +186,7 @@ instance forall sess. P.PersistFieldSql (Decomposed sess) => P.PersistEntity (Pe
         []
         True
         P.NoReference
+        Nothing
   persistFieldDef PersistentSessionAccessedAt
     = P.FieldDef
         (P.HaskellName "accessedAt")
@@ -189,6 +196,7 @@ instance forall sess. P.PersistFieldSql (Decomposed sess) => P.PersistEntity (Pe
         []
         True
         P.NoReference
+        Nothing
 
   persistIdField = PersistentSessionId
 
diff --git a/serversession-backend-redis/serversession-backend-redis.cabal b/serversession-backend-redis/serversession-backend-redis.cabal
index 7281279..584d3bd 100644
--- a/serversession-backend-redis/serversession-backend-redis.cabal
+++ b/serversession-backend-redis/serversession-backend-redis.cabal
@@ -27,7 +27,7 @@ library
   build-depends:
       base                      == 4.*
     , bytestring
-    , hedis                     < 0.11
+    , hedis                     < 0.13
     , path-pieces
     , tagged                    >= 0.7
     , text
diff --git a/serversession/serversession.cabal b/serversession/serversession.cabal
index e0a38e5..d9ce51c 100644
--- a/serversession/serversession.cabal
+++ b/serversession/serversession.cabal
@@ -29,6 +29,7 @@ library
     , hashable
     , nonce                     == 1.0.*
     , path-pieces
+    , persistent-test
     , text
     , time
     , transformers
diff --git a/serversession/src/Web/ServerSession/Core/StorageTests.hs b/serversession/src/Web/ServerSession/Core/StorageTests.hs
index 875d1b1..b7e7fda 100644
--- a/serversession/src/Web/ServerSession/Core/StorageTests.hs
+++ b/serversession/src/Web/ServerSession/Core/StorageTests.hs
@@ -10,6 +10,7 @@ module Web.ServerSession.Core.StorageTests
 import Control.Applicative as A
 import Control.Exception (Exception)
 import Control.Monad
+import DataTypeTest (roundUTCTime)
 import Web.ServerSession.Core.Internal
 
 import qualified Crypto.Nonce as N
@@ -173,8 +174,8 @@ allStorageTests storage it runIO parallel _shouldBe shouldReturn shouldThrow = d
               { sessionKey        = sid
               , sessionAuthId     = Nothing
               , sessionData       = SessionMap $ HM.fromList vals
-              , sessionCreatedAt  = now
-              , sessionAccessedAt = now
+              , sessionCreatedAt  = roundUTCTime now
+              , sessionAccessedAt = roundUTCTime now
               }
             ver2 = session { sessionData = SessionMap HM.empty }
         run (getSession storage sid) `shouldReturn` Nothing
@@ -223,8 +224,8 @@ generateSession gen hasAuthId = do
     { sessionKey        = sid
     , sessionAuthId     = authId
     , sessionData       = SessionMap data_
-    , sessionCreatedAt  = TI.addUTCTime (-1000) now
-    , sessionAccessedAt = now
+    , sessionCreatedAt  = roundUTCTime $ TI.addUTCTime (-1000) now
+    , sessionAccessedAt = roundUTCTime $ now
     }
 
 data HasAuthId = HasAuthId | NoAuthId
diff --git a/stack.yaml b/stack.yaml
index fd71006..055ac39 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -1,4 +1,18 @@
-resolver: lts-12.13
+flags:
+  serversession-backend-acid-state:
+    lib-Werror: true
+  serversession-frontend-wai:
+    lib-Werror: true
+  serversession-backend-persistent:
+    lib-Werror: true
+  serversession:
+    lib-Werror: true
+  serversession-backend-redis:
+    lib-Werror: true
+  serversession-frontend-yesod:
+    lib-Werror: true
+  serversession-frontend-snap:
+    lib-Werror: true
 packages:
   - serversession
   - serversession-backend-acid-state
@@ -8,25 +22,11 @@ packages:
   - serversession-frontend-wai
   - serversession-frontend-yesod
   - examples/serversession-example-yesod-persistent
-flags:
-  serversession:
-    lib-Werror: true
-  serversession-backend-acid-state:
-    lib-Werror: true
-  serversession-backend-persistent:
-    lib-Werror: true
-  serversession-backend-redis:
-    lib-Werror: true
-  serversession-frontend-snap:
-    lib-Werror: true
-  serversession-frontend-wai:
-    lib-Werror: true
-  serversession-frontend-yesod:
-    lib-Werror: true
 extra-deps:
-  - acid-state-0.14.3
-  - snap-1.1.1.0
-  - heist-1.1
-  - map-syntax-0.3
-  - pwstore-fast-2.4.4
-  - xmlhtml-0.2.5.2
+  - snap-1.1.3.0@sha256:0c0814d2ab1c3d5f22cef4615b2913ded18e87710ea6febdbe6e3ab8d9838735,8872
+  - acid-state-0.16.0@sha256:a5640fd8d99bdb5f152476a2ae56cc8eb81864b280c8ec7d1387e81296ed844d,6190
+  - heist-1.1.0.1@sha256:7355cf8c1a7ef84c0c800c0539f7b99e7e540cace697238009a2dea1f05231f1,9311
+  - map-syntax-0.3@sha256:ca8b449615fa57419c16a5e98844624a6ac758692b87b3cfae8c74c87c56f1b2,2420
+  - pwstore-fast-2.4.4@sha256:9b6a37510d8b9f37f409a8ab3babac9181afcaaa3fce8ba1c131a7ed3de30698,1351
+  - xmlhtml-0.2.5.2@sha256:3f0990f725551985d777e8edb5563fe99aee998f1fde6a7633f720f76df54701,46997
+resolver: lts-15.8
diff --git a/stack.yaml.lock b/stack.yaml.lock
new file mode 100644
index 0000000..3da7d60
--- /dev/null
+++ b/stack.yaml.lock
@@ -0,0 +1,54 @@
+# This file was autogenerated by Stack.
+# You should not edit this file by hand.
+# For more information, please see the documentation at:
+#   https://docs.haskellstack.org/en/stable/lock_files
+
+packages:
+- completed:
+    hackage: snap-1.1.3.0@sha256:0c0814d2ab1c3d5f22cef4615b2913ded18e87710ea6febdbe6e3ab8d9838735,8872
+    pantry-tree:
+      size: 5743
+      sha256: 804f55a8cab81e720547308e799243e81f43e089b860c0d3160a938cad86ed0d
+  original:
+    hackage: snap-1.1.3.0@sha256:0c0814d2ab1c3d5f22cef4615b2913ded18e87710ea6febdbe6e3ab8d9838735,8872
+- completed:
+    hackage: acid-state-0.16.0@sha256:a5640fd8d99bdb5f152476a2ae56cc8eb81864b280c8ec7d1387e81296ed844d,6190
+    pantry-tree:
+      size: 13678
+      sha256: c6e4b7f00d2a500e6286beafe3a2da7ba898a9ea31f5744df57cdce8a8f5890f
+  original:
+    hackage: acid-state-0.16.0@sha256:a5640fd8d99bdb5f152476a2ae56cc8eb81864b280c8ec7d1387e81296ed844d,6190
+- completed:
+    hackage: heist-1.1.0.1@sha256:7355cf8c1a7ef84c0c800c0539f7b99e7e540cace697238009a2dea1f05231f1,9311
+    pantry-tree:
+      size: 7354
+      sha256: a2635ed49de6debaf8b98189989f83ab58dcc125c6ae8e57f6fe0903bc7fa8ff
+  original:
+    hackage: heist-1.1.0.1@sha256:7355cf8c1a7ef84c0c800c0539f7b99e7e540cace697238009a2dea1f05231f1,9311
+- completed:
+    hackage: map-syntax-0.3@sha256:ca8b449615fa57419c16a5e98844624a6ac758692b87b3cfae8c74c87c56f1b2,2420
+    pantry-tree:
+      size: 558
+      sha256: c196bee0433f9540e4251ebd1be06d802ff7cc4931e6f0aedc38babd28683a3c
+  original:
+    hackage: map-syntax-0.3@sha256:ca8b449615fa57419c16a5e98844624a6ac758692b87b3cfae8c74c87c56f1b2,2420
+- completed:
+    hackage: pwstore-fast-2.4.4@sha256:9b6a37510d8b9f37f409a8ab3babac9181afcaaa3fce8ba1c131a7ed3de30698,1351
+    pantry-tree:
+      size: 270
+      sha256: ff4a44ede62515efe5cd366a5803f7183c811c4a0cf56eea88da94181c4844c0
+  original:
+    hackage: pwstore-fast-2.4.4@sha256:9b6a37510d8b9f37f409a8ab3babac9181afcaaa3fce8ba1c131a7ed3de30698,1351
+- completed:
+    hackage: xmlhtml-0.2.5.2@sha256:3f0990f725551985d777e8edb5563fe99aee998f1fde6a7633f720f76df54701,46997
+    pantry-tree:
+      size: 61835
+      sha256: bb1bd95db3738e18d112bbc9724510ee64a51b7eda61494507f4957c5e2281f6
+  original:
+    hackage: xmlhtml-0.2.5.2@sha256:3f0990f725551985d777e8edb5563fe99aee998f1fde6a7633f720f76df54701,46997
+snapshots:
+- completed:
+    size: 492015
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/15/8.yaml
+    sha256: 926bc3d70249dd0ba05277ff00943c0addb35b627cb641752669e7cf771310d0
+  original: lts-15.8