When npm shrinkwrap fails

I was getting this error:

npm ERR! Darwin 15.2.0
npm ERR! argv "/Users/justin/.nvm/versions/node/v5.0.0/bin/node" "/Users/justin/.nvm/versions/node/v5.0.0/bin/npm" "shrinkwrap"
npm ERR! node v5.0.0
npm ERR! npm  v3.5.0

npm ERR! Problems were encountered
npm ERR! Please correct and try again.
npm ERR! extraneous: sleep@3.0.0 /Users/justin/shakacode/react-webpack-rails-tutorial/client/node_modules/sleep
npm ERR!
npm ERR! If you need help, you may report this error at:
npm ERR!     <https://github.com/npm/npm/issues>

npm ERR! Please include the following file with any support request:
npm ERR!     /Users/justin/shakacode/react-webpack-rails-tutorial/client/npm-debug.log

This fixed it for me:

npm uninstall --save-dev sleep && npm i --save-dev sleep && npm prune && npm shrinkwrap

Another thing to try is:

npm cache clean

And be sure you have relatively current versions of npm and maybe node.