diff --git a/static/js/muting.js b/static/js/muting.js
index a7319b38ed..30a43c8d52 100644
--- a/static/js/muting.js
+++ b/static/js/muting.js
@@ -21,6 +21,9 @@ exports.unmute_topic = function (stream, topic) {
 };
 
 exports.is_topic_muted = function (stream, topic) {
+    if (stream === undefined) {
+        return false;
+    }
     var sub_dict = muted_topics.get(stream);
     return sub_dict && sub_dict.get(topic);
 };
diff --git a/zerver/tests/frontend/node/muting.js b/zerver/tests/frontend/node/muting.js
index 631267829a..5f60ac5d4c 100644
--- a/zerver/tests/frontend/node/muting.js
+++ b/zerver/tests/frontend/node/muting.js
@@ -11,6 +11,14 @@ set_global('page_params', {
 
 var muting = require('js/muting.js');
 
+(function test_edge_cases() {
+    // private messages
+    assert(!muting.is_topic_muted(undefined, undefined));
+
+    // defensive
+    assert(!muting.is_topic_muted('nonexistent', undefined));
+}());
+
 (function test_basics() {
     assert(!muting.is_topic_muted('devel', 'java'));
     muting.mute_topic('devel', 'java');