Crash when connecting

I'm trying to follow along the example. I start a socket repl, and eval the following lines:

(require '[trikl.core :as t])

;; store clients so we can poke at them from the REPL
(def clients (atom []))

;; Start the server on port 1357, as an accept handler just store the client in
;; the atom.
(def stop-server (t/start-server #(swap! clients conj %) 1357))

Then when connecting using telnet localhost 1357 I get the following exception in the repl window:

Exception in server loop #error {
 :cause class java.net.Socket$SocketOutputStream cannot be cast to class java.io.FileOutputStream (java.net.Socket$SocketOutputStream and java.io.FileOutputStream are in module java.base of loader 'bootstrap')
 :via
 [{:type java.lang.ClassCastException
   :message class java.net.Socket$SocketOutputStream cannot be cast to class java.io.FileOutputStream (java.net.Socket$SocketOutputStream and java.io.FileOutputStream are in module java.base of loader 'bootstrap')
   :at [trikl.telnet$send_telnet_command invokeStatic telnet.clj 31]}]
 :trace
 [[trikl.telnet$send_telnet_command invokeStatic telnet.clj 31]
  [trikl.telnet$send_telnet_command doInvoke telnet.clj 31]
  [clojure.lang.RestFn invoke RestFn.java 2442]
  [trikl.telnet$prep_telnet invokeStatic telnet.clj 39]
  [trikl.telnet$prep_telnet invoke telnet.clj 38]
  [trikl.core$start_client invokeStatic core.clj 526]
  [trikl.core$start_client invoke core.clj 524]
  [trikl.core$make_client invokeStatic core.clj 557]
  [trikl.core$make_client doInvoke core.clj 547]
  [clojure.lang.RestFn invoke RestFn.java 442]
  [trikl.core$start_server$fn__599 invoke core.clj 625]
  [clojure.core$binding_conveyor_fn$fn__5754 invoke core.clj 2030]
  [clojure.lang.AFn call AFn.java 18]
  [java.util.concurrent.FutureTask run FutureTask.java 264]
  [java.util.concurrent.ThreadPoolExecutor runWorker ThreadPoolExecutor.java 1128]
  [java.util.concurrent.ThreadPoolExecutor$Worker run ThreadPoolExecutor.java 628]
  [java.lang.Thread run Thread.java 830]]}

I'm on MacOS. Have tried connecting using eshell and shell in emacs, and zsh terminal.

Start with trikl/telnet.clj at send_telnet_command, line 31, then follow the prep_telnet and start_client calls shown in the stack trace. Reproduce the example with start-server and telnet localhost 1357; done means a client connection no longer crashes the server loop on macOS shells.