Bug with using underscores in folder names

[vuk]

I'm using Mocha with Istanbul and I have a module loaded globally to Mocha with --require test/common
In package.json I have posttest script that runs Istanbul to generate coverage report.

istanbul cover node_modules/mocha/bin/_mocha -- -R spec --recursive

Now, I tried to organize my tests to follow my app folder structure. So i have something like this

-- app
------ __lib__
------ another folder

So this structure should be followed in test folder also.
But when I add files to lib folder I get an error about global.my_helper that I've loaded from common module saying Object #<Object> has no method 'my_helper'.

When I rename __lib__ to lib everything goes as expected.

Also Mocha passes all tests even with __lib__, but Istanbul crashes and doesn't generate coverage report