npm install -g mern cli -> can't install

(Mac OS X 10.13.6) prefix with sudo, install hangs with

Unhandled rejection Error: Command failed: /usr/bin/git clone --depth=1 -q -b 0.0.2 git://github.com/timoxley/next-tick.git /Users/jon/.npm/_cacache/tmp/git-clone-bc41fa44
/Users/jon/.npm/_cacache/tmp/git-clone-bc41fa44/.git: Permission denied

at ChildProcess.exithandler (child_process.js:291:12)
at ChildProcess.emit (events.js:182:13)
at maybeClose (internal/child_process.js:961:16)
at Socket.stream.socket.on (internal/child_process.js:380:11)
at Socket.emit (events.js:182:13)
at Pipe._handle.close [as _onclose] (net.js:595:12)

Can't understand the "Permission denied" as path to .../tmp exists and is owned by me ?