diff --git a/doc/contributions.txt b/doc/contributions.txt
index f7dada053be09207533abc6284dc8d0a590c7d3c..b49a3649d5fca9c677b9e9fadf278919b53f1d9a 100644
--- a/doc/contributions.txt
+++ b/doc/contributions.txt
@@ -150,6 +150,7 @@ blino Nakamura
 	VWR-17
 Boroondas Gupte
 	SNOW-278
+	SNOW-527
 	VWR-233
 	WEB-262
 Bulli Schumann
diff --git a/indra/lib/python/indra/ipc/llmessage.py b/indra/lib/python/indra/ipc/llmessage.py
index 6161badc70632abe113467390aa057d444209b20..91fb36b72c62b4979c9ca1cde2f3f4ab00fc2833 100644
--- a/indra/lib/python/indra/ipc/llmessage.py
+++ b/indra/lib/python/indra/ipc/llmessage.py
@@ -26,8 +26,6 @@ THE SOFTWARE.
 $/LicenseInfo$
 """
 
-from sets import Set, ImmutableSet
-
 from compatibility import Incompatible, Older, Newer, Same
 from tokenstream import TokenStream
 
@@ -44,8 +42,8 @@ class Template:
     
     def compatibleWithBase(self, base):
         messagenames = (
-              ImmutableSet(self.messages.keys())
-            | ImmutableSet(base.messages.keys())
+              frozenset(self.messages.keys())
+            | frozenset(base.messages.keys())
             )
             
         compatibility = Same()