With a reinstall of node/npm and clean node_modules I am able to replicate the error (this happens only w/ the prosemirror package and not other node packages), but I’m not an node expert so if no one else has reported problems, then ¯\_(ツ)_/¯.
I was able to work around the issue by using the package.json from this thread which pulls directly from the git repo, so if using npm is preferred over bower, that’s what I’ll use.
For those interested, here’s how I ended up building the prosemirror code: https://github.com/lhl/pensieve/tree/master/js