[racket] Simple json web response

I'm new to racket -- so don't assume I know what I am doing. I tried
running this code, and the web server just hangs. I am just trying to
get a racket server to return a static json file. Any tips on how to
get started would be appreciated.

#lang web-server/insta

(define (start request)
  (response/xexpr #:mime-type #"application/json"
                  "{1: 11}"))