Issue #16856: Fix a segmentation fault from calling repr() on a dict with

a key whose repr raise an exception.

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.4.0 Alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #16856: Fix a segmentation fault from calling repr() on a dict with
+  a key whose repr raise an exception.
+
 - Issue #16367: Fix FileIO.readall() on Windows for files larger than 2 GB.
 
 - Issue #16761: Calling int() with base argument only now raises TypeError.

Objects/unicodeobject.c

@@ -10548,7 +10548,7 @@ PyUnicode_Append(PyObject **p_left, PyObject *right)
         return;
     }
     left = *p_left;
-    if (right == NULL || !PyUnicode_Check(left)) {
+    if (right == NULL || left == NULL || !PyUnicode_Check(left)) {
         if (!PyErr_Occurred())
             PyErr_BadInternalCall();
         goto error;