Cannot find module 'debug'
I'm getting this error:
```
Error: Cannot find module 'debug'
    at Function.Module._resolveFilename (module.js:325:15)
    at Function.Module._load (module.js:276:25)
    at Module.require (module.js:353:17)
    at require (internal/module.js:12:17)
    at Object.<anonymous> (myapp/node_modules/koa/lib/application.js:8:13)
    at Module._compile (module.js:409:26)
    at Object.Module._extensions..js (module.js:416:10)
    at Module.load (module.js:343:32)
    at Function.Module._load (module.js:300:12)
    at Module.require (module.js:353:17)
```
The koa's version in use is the 1.2.4.

Are there someone having this problem too?