diff --git a/openmc/statepoint.py b/openmc/statepoint.py
index 6c8af88a7..19aa3dbaf 100644
--- a/openmc/statepoint.py
+++ b/openmc/statepoint.py
@@ -622,8 +622,6 @@ class StatePoint(object):
 
         Raises
         ------
-        RuntimeError
-            If a Summary object has already been linked.
         ValueError
             An error when the argument passed to the 'summary' parameter is not
             an openmc.Summary object.
@@ -631,7 +629,9 @@ class StatePoint(object):
         """
 
         if self.summary is not None:
-            raise RuntimeError('A Summary object has already been linked.')
+            warnings.warn('A Summary object has already been linked.',
+                          RuntimeWarning)
+            return
 
         if not isinstance(summary, openmc.summary.Summary):
             msg = 'Unable to link statepoint with "{0}" which ' \
diff --git a/openmc/universe.py b/openmc/universe.py
index 8834eaa52..770e789da 100644
--- a/openmc/universe.py
+++ b/openmc/universe.py
@@ -36,8 +36,8 @@ class Universe(object):
         automatically be assigned
     name : str, optional
         Name of the universe. If not specified, the name is the empty string.
-    cells : Iterable of openmc.Cell
-        Cells to add to the universe
+    cells : Iterable of openmc.Cell, optional
+        Cells to add to the universe. By default no cells are added.
 
     Attributes
     ----------