npm adduser fails with 404 not found - npm

Question

I'm trying to publish to a private registry, but I need to add a user first. When I try:
npm adduser --registry=http://my-registry:8081
I get the following prompt/output:
Username: rjohns
Password:
Email: (this IS public) rjohns#email.com
npm ERR! code E404
npm ERR! 404 Not Found
npm ERR! A complete log of this run can be found in:
npm ERR! /Users/rjohns/.npm/_logs/2018-02-09T23_30_02_911Z-debug.log
The log mentioned shows:
0 info it worked if it ends with ok
1 verbose cli [ '/usr/local/bin/node',
1 verbose cli '/usr/local/bin/npm',
1 verbose cli 'adduser',
1 verbose cli '--registry=http://my-registry:8081' ]
2 info using npm#5.5.1
3 info using node#v8.9.3
4 verbose stack Error: Not Found
4 verbose stack at fetch.defaults.then.then.result
(/usr/local/lib/node_modules/npm/node_modules/npm-profile/index.js:203:19)
4 verbose stack at
4 verbose stack at process._tickCallback
(internal/process/next_tick.js:188:7)
5 verbose cwd /Users/rjohns/Developer/hybrid-plugin copy
6 verbose Darwin 16.7.0
7 verbose argv "/usr/local/bin/node" "/usr/local/bin/npm" "adduser"
"--registry=http://my-registry:8081"
8 verbose node v8.9.3
9 verbose npm v5.5.1
10 error code E404
11 error 404 Not Found
12 verbose exit [ 1, true ]
What is going on here?