The following code causes an assertion failure on Windows:
class BadEnv(dict):
    keys = None

import subprocess

import sys

subprocess.Popen([sys.executable, "-c", "pass"], env=BadEnv())


this is because getenvironment() (in Modules/_winapi.c) calls PyMapping_Values()
immediately after calling PyMapping_Keys().
calling PyMapping_Values() ultimately leads to calling _PyObject_FastCallDict(),
which does 'assert(!PyErr_Occurred());'.
thus, in case of an error in PyMapping_Keys(), the assertion fails.