Bad response for an exception

gistfile1.txt

qrilka@qdesktop ~ $ curl -v localhost:8080/evaluated
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 8080 (#0)
> GET /evaluated HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.59.0
> Accept: */*
> 
* HTTP 1.0, assume close after body
< HTTP/1.0 500 Internal Server Error
< Date: Fri, 27 Apr 2018 08:42:36 GMT
< Server: Warp/3.2.17
< Content-Type: text/plain; charset=utf-8
< 
* Closing connection 0
Something went wrongqrilka@qdesktop ~ $ curl -v localhost:8080/unevaluated
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 8080 (#0)
> GET /unevaluated HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.59.0
> Accept: */*
> 
* Empty reply from server
* Connection #0 to host localhost left intact
curl: (52) Empty reply from server

test.hs

{-# LANGUAGE DataKinds        #-}
{-# LANGUAGE TypeOperators    #-}
{-# OPTIONS_GHC -Wall -Werror #-}
module Main (main) where

import Network.Wai.Handler.Warp (run)
import Servant

type API =
  "evaluated" :> Get '[JSON] String
  :<|>
  "unevaluated" :> Get '[JSON] String

main :: IO ()
main = run 8080 $ serveWithContext (Proxy :: Proxy API) (True :. EmptyContext) server
  where
    server :: Server API
    server = evaluated :<|> unevaluated
    evaluated = error "evaluated"
    unevaluated = return $ error "evaluated"