diff --git a/static/js/subs.js b/static/js/subs.js
index 5b2507a5d8..932c8b3dc4 100644
--- a/static/js/subs.js
+++ b/static/js/subs.js
@@ -120,6 +120,11 @@ function update_stream_notifications(sub, value) {
 }
 
 function update_stream_name(sub, new_name) {
+    if (sub === undefined) {
+        // This isn't a stream we know about, so ignore it.
+        return;
+    }
+
     // Rename the stream internally.
     var old_name = sub.name;
     stream_data.delete_sub(old_name);