diff --git a/Lib/locale.py b/Lib/locale.py
index ce0ca81..448b018 100644
--- a/Lib/locale.py
+++ b/Lib/locale.py
@@ -505,7 +505,11 @@
         def getpreferredencoding(do_setlocale = True):
             """Return the charset that the user is likely using,
             by looking at environment variables."""
-            return getdefaultlocale()[1]
+            res = getdefaultlocale()[1]
+            if res is None:
+                # LANG not set, default conservatively to ASCII
+                res = 'ascii'
+            return res
     else:
         def getpreferredencoding(do_setlocale = True):
             """Return the charset that the user is likely using,
diff --git a/Misc/NEWS b/Misc/NEWS
index 79bdef6..84b3d3a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -14,6 +14,13 @@
 
 - Use wchar_t functions in _locale module.
 
+Library
+-------
+
+- Default to ASCII as the locale.getpreferredencoding, if the POSIX
+  system doesn't support CODESET and LANG isn't set or doesn't
+  allow deduction of an encoding.
+
 
 What's New in Python 3.0a3?
 ===========================