Node application runs on Heroku but not localhost

I have a simple REST API using MEAN. It's hosted on Heroku, and everything works well there. However, if I try to run it on localhost I get the following error:

TypeError: Parameter "url" must be a string, not undefined
at Url.parse (url.js:81:11)
at Object.urlParse [as parse] (url.js:75:5)
at module.exports (/Users/ricardotaboada/Desktop/sbackend/node_modules/mongodb/lib/url_parser.js:15:23)
at connect (/Users/ricardotaboada/Desktop/sbackend/node_modules/mongodb/lib/mongo_client.js:403:16)
at Function.MongoClient.connect (/Users/ricardotaboada/Desktop/sbackend/node_modules/mongodb/lib/mongo_client.js:227:3)
at Object.<anonymous> (/Users/ricardotaboada/Desktop/sbackend/server.js:15:21)
at Module._compile (module.js:571:32)
at Object.Module._extensions..js (module.js:580:10)
at Module.load (module.js:488:32)
at tryModuleLoad (module.js:447:12)

Any idea what's going on? Thanks very much.

via R. Taboada