I get this error if I run
curl http://npmjs.org/install.sh | sh
even with sudo.
cirk@cirk-Parallels-Virtual-Platform:~$ cu
Why are you running that in /usr?
/usr
If you really need to, this page suggests you may run that as root, so that would be:
curl https://www.npmjs.com/install.sh | sudo sh
But you better make sure it won't break your system, before running that.