diff --git a/lib/media/media_source_engine.js b/lib/media/media_source_engine.js
index 0550b6667e..1531bd6157 100644
--- a/lib/media/media_source_engine.js
+++ b/lib/media/media_source_engine.js
@@ -271,7 +271,7 @@ shaka.media.MediaSourceEngine = class {
    * @param {string} mimeType
    */
   addSecondarySource(uri, mimeType) {
-    if (!this.video_ || !(this.mediaSource_ instanceof ManagedMediaSource)) {
+    if (!this.video_ || !window.ManagedMediaSource || !this.mediaSource_) {
       shaka.log.warning(
           'Secondary source is used only with ManagedMediaSource');
       return;