fix subprocess on Windows (#391)

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 23e9bd328c12818f0a022e9ae8fccda3e6e83dff..76c340c87970aa80a592a7c08f0ac23884a2cd6b 100644 (file)
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -988,7 +988,7 @@ class Popen(object):
                                          int(not close_fds),
                                          creationflags,
                                          env,
-                                         os.fspath(cwd),
+                                         os.fspath(cwd) if cwd is not None else None,
                                          startupinfo)
             finally:
                 # Child is launched. Close the parent's copy of those pipe