diff -r 51ac5f06dd04 Lib/test/test_os.py
--- a/Lib/test/test_os.py	Tue Jul 24 03:45:39 2012 -0700
+++ b/Lib/test/test_os.py	Wed Jul 25 11:35:41 2012 +0900
@@ -1047,6 +1047,9 @@
     def test_chdir(self):
         self.assertRaises(WindowsError, os.chdir, support.TESTFN)
 
+    def test_chdir_invalid_filename(self):
+        self.assertRaises(WindowsError, os.chdir, b'\xe7w\xf0')
+
     def test_mkdir(self):
         f = open(support.TESTFN, "w")
         try:
diff -r 51ac5f06dd04 Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Tue Jul 24 03:45:39 2012 -0700
+++ b/Objects/unicodeobject.c	Wed Jul 25 11:35:41 2012 +0900
@@ -3694,7 +3694,7 @@
 PyUnicode_DecodeFSDefaultAndSize(const char *s, Py_ssize_t size)
 {
 #ifdef HAVE_MBCS
-    return PyUnicode_DecodeMBCS(s, size, NULL);
+    return PyUnicode_DecodeMBCS(s, size, "surrogateescape");
 #elif defined(__APPLE__)
     return PyUnicode_DecodeUTF8Stateful(s, size, "surrogateescape", NULL);
 #else