Logging module script: using os.getenv

Question

I am learning Python and am following the book A byte of python. In an exercise talking about the logging module there is the following line of code:

if platform.system().startswith('Windows'):
logging_file = os.path.join(os.getenv('DIRETORIOINICIAL'), os.getenv('PATH'), 'test.log')

The script just writes inside the file teste.log with the date and time the script was run.

I followed exactly as it is in the book but I always get the same trackback:

Traceback (most recent call last):
  File "using_logging.py", line 6, in <module>
    logging_file = os.path.join(os.getenv('DIRETÓRIOINICIAL'), os.getenv('PATH'), 'test.log')
  File "C:\Python34\lib\ntpath.py", line 108, in join
    result_drive, result_path = splitdrive(path)
  File "C:\Python34\lib\ntpath.py", line 159, in splitdrive
    if len(p) > 1:
TypeError: object of type 'NoneType' has no len()

It does not return or write the file.