The bufsize argument to Popen() should accept None meaning the default (0).
diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 051f2d4..1ae7426 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -465,6 +465,8 @@
         _cleanup()
 
         self._child_created = False
+        if bufsize is None:
+            bufsize = 0  # Restore default
         if not isinstance(bufsize, int):
             raise TypeError("bufsize must be an integer")