The addition of the _posixsubprocess module in 3.2 introduced a change of behaviour when passing an empty dict (or other false value besides None) as env:

python3.1 -c 'import subprocess; print(subprocess.Popen(["env"], env={}, stdout=subprocess.PIPE).communicate())'
(b'', None)

python3.2 -c 'import subprocess; print(subprocess.Popen(["env"], env={}, stdout=subprocess.PIPE).communicate())'
(b'LC_MONETARY=C\nSHELL=/bin/bash\nTERM=scr....', None)

The test for 'env' being the default (None) is not explicit enough.