Very strange thing happened to me.
I was running beta 16 just fine, I have a deployed app running 16 and its working great.
All of a sudden I started to get this error.
About an hour ago I got this error.
Upgraded to rc2 and I am still getting it. 
While processing files with cosmos:browserify (for target web.browser):
packages/kadira:flow-router/client.browserify.js: Arguments to path.resolve must be strings
Browserify options:
> undefined
packages/kadira:dochead/package.browserify.js: Arguments to path.resolve must be strings
Browserify options:
> undefined