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 @@
 $/LicenseInfo$
 """
 
-from sets import Set, ImmutableSet
-
 from compatibility import Incompatible, Older, Newer, Same
 from tokenstream import TokenStream
 
@@ -44,8 +42,8 @@ def addMessage(self, m):
     
     def compatibleWithBase(self, base):
         messagenames = (
-              ImmutableSet(self.messages.keys())
-            | ImmutableSet(base.messages.keys())
+              frozenset(self.messages.keys())
+            | frozenset(base.messages.keys())
             )
             
         compatibility = Same()