Underscore with nodejs and forever

I have a nodejs app and a module in a /lib folder that requires underscore. When I run nodejs, everything works fine but when I try to use forever with this command: NODE_ENV=production forever start -l forever.log -o out.log -e err.log app.js I have this error in forever.log:

module.js:340
    throw err;
      ^
Error: Cannot find module 'underscore'
    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> (/var/www/nodejs_app/lib/utils.js:1:71)
    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)
error: Forever detected script exited with code: 8