Since I was using git submodules, the path @app/express-base was pointing to that sub module which wasn't initialized in production. I had already initialized it in dev, that's why it was working locally.

In prod, I needed to run git submodule update --remote in order to initialize it.

If you didn't get anything in all I just said, then I summarize it as, "make sure your path value is pointing to a path that actually EXISTS"