What is the proper way of dealing with unicode subprocess env's in Windows?

Take this example snippet.

import subprocess
import os

env = os.environ.copy()
env["FOO"] = u"foo"
subprocess.check_call(["ls", "-l"], env=env)

On Windows, this fails.

C:\Python27\python.exe test.py
Traceback (most recent call last):
  File "test.py", line 7, in <module>
    subprocess.check_call(["ls", "-l"], env=env)
  File "C:\Python27\lib\subprocess.py", line 535, in check_call
    retcode = call(*popenargs, **kwargs)
  File "C:\Python27\lib\subprocess.py", line 522, in call
    return Popen(*popenargs, **kwargs).wait()
  File "C:\Python27\lib\subprocess.py", line 710, in __init__
    errread, errwrite)
  File "C:\Python27\lib\subprocess.py", line 958, in _execute_child
    startupinfo)
TypeError: environment can only contain strings

sys.path is documented to be perfectly ok with unicode. What is the correct way to deal with this (and similar code) so that everything works as expected? The obvious solution is to call .encode() on the unicode path but I'm not sure if that will lead to unexpected behaviours.