diff --git a/yesod-bin/Build.hs b/yesod-bin/Build.hs
index 3050a1a8..aca37e74 100644
--- a/yesod-bin/Build.hs
+++ b/yesod-bin/Build.hs
@@ -20,8 +20,7 @@ import           Data.Text.Encoding.Error (lenientDecode)
 import           Data.ByteString (ByteString)
 import qualified Data.ByteString as S
 
-import           Control.Exception (SomeException, try, IOException)
-import           Control.Exception.Lifted (handle)
+import           UnliftIO (SomeException, try, IOException, handle)
 import           Control.Monad (when, filterM, forM, forM_, (>=>))
 import           Control.Monad.Trans.State (StateT, get, put, execStateT)
 import           Control.Monad.Trans.Writer (WriterT, tell, execWriterT)
diff --git a/yesod-core/Yesod/Core/Handler.hs b/yesod-core/Yesod/Core/Handler.hs
index 7ff66627..45e15a20 100644
--- a/yesod-core/Yesod/Core/Handler.hs
+++ b/yesod-core/Yesod/Core/Handler.hs
@@ -193,6 +193,7 @@ import           Control.Applicative           ((<$>))
 import           Data.Monoid                   (mempty, mappend)
 #endif
 import           Control.Applicative           ((<|>))
+import qualified Data.CaseInsensitive          as CI
 import           Control.Exception             (evaluate, SomeException, throwIO)
 import           Control.Exception             (handle)
 
@@ -779,7 +780,7 @@ setLanguage = setSession langKey
 --
 -- @since 1.2.0
 addHeader :: MonadHandler m => Text -> Text -> m ()
-addHeader a = addHeaderInternal . Header (encodeUtf8 a) . encodeUtf8
+addHeader a = addHeaderInternal . Header (CI.mk $ encodeUtf8 a) . encodeUtf8
 
 -- | Deprecated synonym for addHeader.
 setHeader :: MonadHandler m => Text -> Text -> m ()
@@ -797,10 +798,10 @@ replaceOrAddHeader :: MonadHandler m => Text -> Text -> m ()
 replaceOrAddHeader a b =
   modify $ \g -> g {ghsHeaders = replaceHeader (ghsHeaders g)}
   where
-    repHeader = Header (encodeUtf8 a) (encodeUtf8 b)
+    repHeader = Header (CI.mk $ encodeUtf8 a) (encodeUtf8 b)
 
     sameHeaderName :: Header -> Header -> Bool
-    sameHeaderName (Header n1 _) (Header n2 _) = T.toLower (decodeUtf8 n1) == T.toLower (decodeUtf8 n2)
+    sameHeaderName (Header n1 _) (Header n2 _) = n1 == n2
     sameHeaderName _ _ = False
 
     replaceIndividualHeader :: [Header] -> [Header]
diff --git a/yesod-core/Yesod/Core/Internal/Response.hs b/yesod-core/Yesod/Core/Internal/Response.hs
index 7e2be331..abf0cdfa 100644
--- a/yesod-core/Yesod/Core/Internal/Response.hs
+++ b/yesod-core/Yesod/Core/Internal/Response.hs
@@ -8,7 +8,6 @@ import qualified Data.ByteString              as S
 import qualified Data.ByteString.Char8        as S8
 import qualified Data.ByteString.Lazy         as BL
 import           Data.CaseInsensitive         (CI)
-import qualified Data.CaseInsensitive         as CI
 import           Network.Wai
 import           Control.Monad                (mplus)
 import           Control.Monad.Trans.Resource (runInternalState, InternalState)
@@ -92,7 +91,7 @@ headerToPair (DeleteCookie key path) =
         , "; expires=Thu, 01-Jan-1970 00:00:00 GMT"
         ]
     )
-headerToPair (Header key value) = (CI.mk key, value)
+headerToPair (Header key value) = (key, value)
 
 evaluateContent :: Content -> IO (Either ErrorResponse Content)
 evaluateContent (ContentBuilder b mlen) = handle f $ do
diff --git a/yesod-core/Yesod/Core/Types.hs b/yesod-core/Yesod/Core/Types.hs
index 78ec0fef..191ea460 100644
--- a/yesod-core/Yesod/Core/Types.hs
+++ b/yesod-core/Yesod/Core/Types.hs
@@ -24,6 +24,7 @@ import           Control.Monad.Logger               (LogLevel, LogSource,
 import           Control.Monad.Trans.Resource       (MonadResource (..), InternalState, runInternalState, MonadThrow (..), throwM, ResourceT)
 import           Data.ByteString                    (ByteString)
 import qualified Data.ByteString.Lazy               as L
+import           Data.CaseInsensitive               (CI)
 import           Data.Conduit                       (Flush, ConduitT)
 import           Data.IORef                         (IORef, modifyIORef')
 import           Data.Map                           (Map, unionWith)