diff --git a/yesod-core/Yesod/Widget.hs b/yesod-core/Yesod/Widget.hs
index 8536d0bf..141a8f2f 100644
--- a/yesod-core/Yesod/Widget.hs
+++ b/yesod-core/Yesod/Widget.hs
@@ -4,6 +4,7 @@
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TypeSynonymInstances #-}
+{-# LANGUAGE UndecidableInstances #-}
 -- | Widgets combine HTML with JS and CSS dependencies with a unique identifier
 -- generator, allowing you to create truly modular HTML components.
 module Yesod.Widget