Servant QueryParams parse error

Given the following code:

newtype HelloMessage = HelloMessage { msg :: String }
deriving (Generic)

instance ToJSON HelloMessage

type API2 = "hello"
:> QueryParam "age" Int
:> Get '[JSON] HelloMessage

appAPI2 :: Proxy API2
appAPI2 = Proxy

myHandler :: Server API2
myHandler = helloHandler
where
helloHandler :: Maybe Int -> Handler HelloMessage
helloHandler mAge =
let
sAge = case mAge of
Nothing -> "0"
Just ag -> show ag
in
return . HelloMessage $ sAge

app2 :: Application
app2 = serve appAPI2 myHandler

main :: IO ()
main = run 8080 app2


I can access /hello which returns { "msg" : 0} or /hello?age=20 which returns { "msg" : 20}. But if I set the age to be a non-integer parseable e.g. "foobar" which makes the url to be /hello?age=foobar and access it, it returns an error message about parsing error on "foobar".

/hello

{ "msg" : 0}

/hello?age=20

{ "msg" : 20}

"foobar"

/hello?age=foobar

"foobar"

This behave differently with Capture where if I give the same treatment it'll just return a http 400.

Capture

What is wrong my code?

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