Index: Python/errors.c
===================================================================
--- Python/errors.c	(revision 65931)
+++ Python/errors.c	(working copy)
@@ -65,6 +65,11 @@
 	Py_XINCREF(value);
 	if (tstate->exc_value != NULL && tstate->exc_value != Py_None) {
 		/* Implicit exception chaining */
+
+		/* Save the current value, as code below may (incorrectly) modify it. */
+		PyObject *exc_value = tstate->exc_value;
+		Py_INCREF(exc_value);
+
 		if (value == NULL || !PyExceptionInstance_Check(value)) {
 			/* We must normalize the value right now */
 			PyObject *args, *fixed_value;
@@ -88,8 +93,8 @@
 		   This is O(chain length) but context chains are
 		   usually very short. Sensitive readers may try
 		   to inline the call to PyException_GetContext. */
-		if (tstate->exc_value != value) {
-			PyObject *o = tstate->exc_value, *context;
+		if (exc_value != value) {
+			PyObject *o = exc_value, *context;
 			while ((context = PyException_GetContext(o))) {
 				Py_DECREF(context);
 				if (context == value) {
@@ -98,9 +103,12 @@
 				}
 				o = context;
 			}
-			Py_INCREF(tstate->exc_value);
-			PyException_SetContext(value, tstate->exc_value);
+			/* This will steal our reference to exc_value */
+			PyException_SetContext(value, exc_value);
 		}
+		else {
+			Py_DECREF(exc_value);
+		}
 	}
 	if (value != NULL && PyExceptionInstance_Check(value))
 		tb = PyException_GetTraceback(value);