diff --git a/lib/reducers/message-reducer.js b/lib/reducers/message-reducer.js
--- a/lib/reducers/message-reducer.js
+++ b/lib/reducers/message-reducer.js
@@ -103,11 +103,8 @@
   const threads: { [threadID: string]: string[] } = {};
   for (const messageInfo of orderedMessageInfos) {
     const key = messageID(messageInfo);
-    if (!threads[messageInfo.threadID]) {
-      threads[messageInfo.threadID] = [key];
-    } else {
-      threads[messageInfo.threadID].push(key);
-    }
+    threads[messageInfo.threadID] = threads[messageInfo.threadID] || [];
+    threads[messageInfo.threadID].push(key);
   }
   return threads;
 }