Node JS & Ionic issue with Error: Cannot find module 'optimist'

I'm trying to build my first app using the Ionic mobile app framework. But when I try to add a platform, build or emulate the mobile app I get this error:

module.js:340
throw err;
      ^
Error: Cannot find module 'optimist'
at Function.Module._resolveFilename (module.js:338:15)
at Function.Module._load (module.js:280:25)
at Module.require (module.js:364:17)
at require (module.js:380:17)
at Object.<anonymous> (/usr/local/lib/node_modules/ionic/lib/ionic.js:17:12)
at Module._compile (module.js:456:26)
at Object.Module._extensions..js (module.js:474:10)
at Module.load (module.js:356:32)
at Function.Module._load (module.js:312:12)
at Module.require (module.js:364:17)

I've tried to npm install and npm install optimist in the directory of the testApp I created.

Anyone know what is causing this issue and how to fix it?