From e550a8de27a7da0a7584ca09ddab54005126f95c Mon Sep 17 00:00:00 2001
From: Paul Romano <paul.k.romano@gmail.com>
Date: Mon, 10 Aug 2015 06:54:02 +0700
Subject: [PATCH] Fix bug in get_unique_universes introduced by #425

---
 openmc/universe.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/openmc/universe.py b/openmc/universe.py
index 75b673746..cdec833cb 100644
--- a/openmc/universe.py
+++ b/openmc/universe.py
@@ -714,7 +714,8 @@ class Lattice(object):
                         assert isinstance(u, Universe)
                         univs[u._id] = u
 
-        univs[self._outer._id] = self._outer
+        if self._outer is not None:
+            univs[self._outer._id] = self._outer
 
         return univs