diff -r a4afcf93ef7b Lib/os.py
--- a/Lib/os.py	Wed Aug 21 21:13:51 2013 -0400
+++ b/Lib/os.py	Fri Aug 23 17:12:03 2013 +0200
@@ -648,7 +648,7 @@ class _Environ(MutableMapping):
             value = self._data[self.encodekey(key)]
         except KeyError:
             # raise KeyError with the original key value
-            raise KeyError(key)
+            raise KeyError(key) from None
         return self.decodevalue(value)
 
     def __setitem__(self, key, value):