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"