diff --git a/src/Foundation.hs b/src/Foundation.hs
index b1a1f6b97..0d18ad4d5 100644
--- a/src/Foundation.hs
+++ b/src/Foundation.hs
@@ -1114,7 +1114,7 @@ instance Yesod UniWorX where
 
           lift . bracketOnError getMessages (mapM_ $ uncurry Yesod.addMessage) $ \msgs -> do
             Just msgs' <- return . forM msgs $ \(msgState, content) -> Message <$> fromPathPiece msgState <*> return content
-            addCustomHeader HeaderAlerts . decodeUtf8 $ JSON.encode msgs'
+            addCustomHeader HeaderAlerts . decodeUtf8 . urlEncode True . toStrict $ JSON.encode msgs'
 
     -- Since we implement `errorHandler` ourselves we don't need `defaultMessageWidget`
     defaultMessageWidget _title _body = error "defaultMessageWidget: undefined"