Index: Objects/object.c
===================================================================
--- Objects/object.c	(revision 47153)
+++ Objects/object.c	(working copy)
@@ -1920,7 +1920,7 @@
 #endif
 	if (op->ob_refcnt < 0)
 		Py_FatalError("UNREF negative refcnt");
-	if (op == &refchain ||
+	if (op == &refchain || !op->_ob_prev || !op->_ob_next ||
 	    op->_ob_prev->_ob_next != op || op->_ob_next->_ob_prev != op)
 		Py_FatalError("UNREF invalid object");
 #ifdef SLOW_UNREF_CHECK