diff -r bbed36370b08 Lib/test/test_raise.py
--- a/Lib/test/test_raise.py	Thu Jan 12 22:46:19 2012 +0100
+++ b/Lib/test/test_raise.py	Thu Jan 26 14:39:14 2012 -0800
@@ -67,6 +67,17 @@ class TestRaise(unittest.TestCase):
                     raise
         self.assertRaises(KeyError, reraise)
 
+    def test_raise_from_None(self):
+        try:
+            try:
+                raise IndexError()
+            except IndexError as e:
+                raise ValueError() from None
+        except ValueError as exc2:
+            self.assertIsNone(exc2.__context__)
+        else:
+            self.fail("No exception raised")
+
     def test_nested_reraise(self):
         def nested_reraise():
             raise
diff -r bbed36370b08 Python/ceval.c
--- a/Python/ceval.c	Thu Jan 12 22:46:19 2012 +0100
+++ b/Python/ceval.c	Thu Jan 26 14:39:14 2012 -0800
@@ -3528,6 +3528,22 @@ do_raise(PyObject *exc, PyObject *cause)
         else if (PyExceptionInstance_Check(cause)) {
             fixed_cause = cause;
         }
+        else if (cause == Py_None) {
+            /* get rid of previous context by setting the current
+               thread's exception information to the replacement exception */
+            PyThreadState *tstate = PyThreadState_GET();
+            PyObject *old_type, *old_value;
+            old_type = tstate->exc_type;
+            old_value = tstate->exc_value;
+            tstate->exc_type = type;
+            tstate->exc_value = value;
+            Py_INCREF(type);
+            Py_INCREF(value);
+            fixed_cause = NULL;
+            Py_DECREF(cause);
+            Py_DECREF(old_type);
+            Py_DECREF(old_value);
+        }
         else {
             PyErr_SetString(PyExc_TypeError,
                             "exception causes must derive from "