python3; ftplib: TypeError: Can't convert 'bytes' object to str implicitly

[Antoon Pardon]

This is the code I run (python 3.3)

host = ...
user = ...
passwd = ...

from ftplib import FTP

ftp = FTP(host, user, passwd)
ftp.mkd(b'NewDir')
ftp.rmd(b'NewDir')

This is the traceback

Traceback (most recent call last):
  File "ftp-problem", line 9, in <module>
    ftp.mkd(b'NewDir')
  File "/usr/lib/python3.3/ftplib.py", line 612, in mkd
    resp = self.voidcmd('MKD ' + dirname)
TypeError: Can't convert 'bytes' object to str implicitly

The problem is that I do something like this in a backup program.
I don't know the locales that other people use. So I manipulate
all file and directory names as bytes.

Am I doing something wrong?

-- 
Antoon Pardon