diff --git a/static/js/message_list.js b/static/js/message_list.js
index 201e4ce607..2e46f49720 100644
--- a/static/js/message_list.js
+++ b/static/js/message_list.js
@@ -227,7 +227,8 @@ MessageList.prototype = {
     },
 
     _is_summarized_message: function (message) {
-        if (message.flags === undefined) {
+        if (!feature_flags.summarize_read_while_narrowed ||
+            message === undefined || message.flags === undefined) {
             return false;
         }
         if (this.summarize_read === 'home') {
diff --git a/zerver/tests/frontend/node/message_list.js b/zerver/tests/frontend/node/message_list.js
index 0184d44493..ad373ef5aa 100644
--- a/zerver/tests/frontend/node/message_list.js
+++ b/zerver/tests/frontend/node/message_list.js
@@ -15,6 +15,7 @@ var assert = require('assert');
 
     global._ = require('third/underscore/underscore.js');
     global.util = require('js/util.js');
+    global.feature_flags = {};
 
 }());