diff -r 222b9392a83b Objects/weakrefobject.c
--- a/Objects/weakrefobject.c	Sat Feb 04 08:05:07 2017 +0200
+++ b/Objects/weakrefobject.c	Sat Feb 04 16:01:08 2017 +0800
@@ -26,6 +26,7 @@
     self->wr_object = ob;
     Py_XINCREF(callback);
     self->wr_callback = callback;
+    self->wr_prev = self->wr_next = NULL;
 }
 
 static PyWeakReference *