Debugging webdriverio tasks

I'm trying to run `grunt-webdriver` as mentioned here:

http://webdriver.io/guide/plugins/grunt-webdriver.html

When debugging, breakpoints aren't working. I've even tried to write "debugger;" inside the code - but it never stops there.  

Debug does work inside `gruntfile.js`, but not inside `spec` files.

Any way to debug the spec files?

EDIT:

My bad. Adding the:

debug: true

To the wdio.conf.js file enables debugging.