debugger: `node --debug-port=1234 debug t.js` doesn't work

[bnoordhuis]

$ cat t.js
debugger;

$ node --debug-port=1234 debug t.js
< Debugger listening on port 5858  # <- this is wrong
debug> . ok
break in t.js:1

# in another window
$ node --debug-port=4321 debug t.js
< Error: listen EADDRINUSE :::5858
<     at Object.exports._errnoException (util.js:874:11)
<     at exports._exceptionWithHostPort (util.js:897:20)
<     at Agent.Server._listen2 (net.js:1234:14)
<     at listen (net.js:1270:10)
<     at Agent.Server.listen (net.js:1366:5)
<     at Object.start (_debug_agent.js:21:9)
<     at startup (node.js:70:9)
<     at node.js:961:3

# in first window
unhandled res:{"seq":4,"request_seq":1,"type":"response","command":"scripts",...

It makes it impossible to debug more than one process at the same time.