diff --git a/Yesod/Core/Class/Yesod.hs b/Yesod/Core/Class/Yesod.hs
--- a/Yesod/Core/Class/Yesod.hs
+++ b/Yesod/Core/Class/Yesod.hs
@@ -266,7 +266,7 @@
     -- Default: the 'defaultYesodMiddleware' function.
     --
     -- Since: 1.1.6
-    yesodMiddleware :: HandlerT site IO res -> HandlerT site IO res
+    yesodMiddleware :: ToTypedContent res => HandlerT site IO res -> HandlerT site IO res
     yesodMiddleware = defaultYesodMiddleware
 
 -- | Default implementation of 'yesodMiddleware'. Adds the response header
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:         1.2.2
+version:         1.2.3
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>