diff --git a/Yesod/EmbeddedStatic.hs b/Yesod/EmbeddedStatic.hs
--- a/Yesod/EmbeddedStatic.hs
+++ b/Yesod/EmbeddedStatic.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
@@ -89,7 +90,11 @@
             resp = case pathInfo req of
                             ("res":_) -> stApp site req
                             ("widget":_) -> staticApp (widgetSettings site) req
+#if MIN_VERSION_wai(3,0,0)
+                            _ -> ($ responseLBS status404 [] "Not Found")
+#else
                             _ -> return $ responseLBS status404 [] "Not Found"
+#endif
 
 -- | Create the haskell variable for the link to the entry
 mkRoute :: ComputedEntry -> Q [Dec]
diff --git a/Yesod/EmbeddedStatic/Internal.hs b/Yesod/EmbeddedStatic/Internal.hs
--- a/Yesod/EmbeddedStatic/Internal.hs
+++ b/Yesod/EmbeddedStatic/Internal.hs
@@ -106,12 +106,22 @@
                  }
     return $ ComputedEntry (ebHaskellName e) st link
 
+toApp :: (Request -> IO Response) -> Application
+#if MIN_VERSION_wai(3, 0, 0)
+toApp f req g = f req >>= g
+#else
+toApp = id
+#endif
+
 tryExtraDevelFiles :: [[T.Text] -> IO (Maybe (MimeType, BL.ByteString))] -> Application
-tryExtraDevelFiles [] _ = return $ responseLBS status404 [] ""
-tryExtraDevelFiles (f:fs) r = do
+tryExtraDevelFiles = toApp . tryExtraDevelFiles'
+
+tryExtraDevelFiles' :: [[T.Text] -> IO (Maybe (MimeType, BL.ByteString))] -> Request -> IO Response
+tryExtraDevelFiles' [] _ = return $ responseLBS status404 [] ""
+tryExtraDevelFiles' (f:fs) r = do
     mct <- liftIO $ f $ drop 1 $ pathInfo r -- drop the initial "res"
     case mct of
-        Nothing -> tryExtraDevelFiles fs r
+        Nothing -> tryExtraDevelFiles' fs r
         Just (mime, ct) -> do
             let hash = T.encodeUtf8 $ T.pack $ base64md5 ct
             let headers = [ ("Content-Type", mime)
@@ -123,11 +133,19 @@
 
 -- | Helper to create the development application at runtime
 develApp :: StaticSettings -> [[T.Text] -> IO (Maybe (MimeType, BL.ByteString))] -> Application
+#if MIN_VERSION_wai(3, 0, 0)
+develApp settings extra req sendResponse = do
+    staticApp settings {ssMaxAge = NoMaxAge} req $ \resp ->
+        if statusCode (responseStatus resp) == 404
+            then tryExtraDevelFiles extra req sendResponse
+            else sendResponse resp
+#else
 develApp settings extra req = do
     resp <- staticApp settings {ssMaxAge = NoMaxAge} req
     if statusCode (responseStatus resp) == 404
         then tryExtraDevelFiles extra req
         else return resp
+#endif
 
 -- | The type of 'addStaticContent'
 type AddStaticContent site = T.Text -> T.Text -> BL.ByteString
diff --git a/yesod-static.cabal b/yesod-static.cabal
--- a/yesod-static.cabal
+++ b/yesod-static.cabal
@@ -1,5 +1,5 @@
 name:            yesod-static
-version:         1.2.3
+version:         1.2.4
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>
@@ -82,6 +82,7 @@
                    , hspec >= 1.3
                    , yesod-test >= 1.2
                    , wai-test
+                   , wai-extra
                    , HUnit
 
                    -- copy from above