Error: Cannot find module 'connect-form' - despite it existing

I've got a railwayjs app

I have installed connect-form with:

$ npm install connect-form

It's also in the package.json file
(for deploying to heroku)

I've checked the node_modules folder, and it's definately there.

When I run the app, I get:

Error: Cannot find module 'connect-form'