Connection to mongo fails in production

I'm sorta new to node and express, but I'm really liking it. I'm working on a website with a mongodb database. It works great, except, when I change the node_env to production, suddenly, it stops writing to mongo, without giving very helpful errors.

Any idea what goes wrong? It works perfect in development mode.

I should add that the database is hosted on MongoHQ, if that makes a difference.