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