Lua os.remove() folder results in 'Permission denied'

I am trying to use os.remove() to delete an empty folder in Windows 7 using Lua but it just results in 'Permission Denied'. I can delete files with no problem.

However if I use os.execute() to call rmdir the folder does get deleted.

I have searched around but cannot find much on this, are there any limitations with os.remove() in a Windows environment?

Thanks.