diff --git a/lib/media/transmuxer.js b/lib/media/transmuxer.js
index 5119c2e255..c6c64c5665 100644
--- a/lib/media/transmuxer.js
+++ b/lib/media/transmuxer.js
@@ -29,9 +29,7 @@ shaka.media.Transmuxer = class {
     this.muxjs_ = shaka.dependencies.muxjs();
 
     /** @private {muxjs.mp4.Transmuxer} */
-    this.muxTransmuxer_ = new this.muxjs_.mp4.Transmuxer({
-      'keepOriginalTimestamps': true,
-    });
+    this.muxTransmuxer_ = null;
 
     /** @private {shaka.util.PublicPromise} */
     this.transmuxPromise_ = null;
@@ -48,16 +46,22 @@ shaka.media.Transmuxer = class {
     /** @private {boolean} */
     this.isTransmuxing_ = false;
 
-    this.muxTransmuxer_.on('data', (segment) => this.onTransmuxed_(segment));
-
-    this.muxTransmuxer_.on('done', () => this.onTransmuxDone_());
+    if (this.muxjs_) {
+      this.muxTransmuxer_ = new this.muxjs_.mp4.Transmuxer({
+        'keepOriginalTimestamps': true,
+      });
+      this.muxTransmuxer_.on('data', (segment) => this.onTransmuxed_(segment));
+      this.muxTransmuxer_.on('done', () => this.onTransmuxDone_());
+    }
   }
 
   /**
    * @override
    */
   destroy() {
-    this.muxTransmuxer_.dispose();
+    if (this.muxTransmuxer_) {
+      this.muxTransmuxer_.dispose();
+    }
     this.muxTransmuxer_ = null;
     return Promise.resolve();
   }
@@ -156,6 +160,8 @@ shaka.media.Transmuxer = class {
    *                     metadata: !Array.<!Object>}>}
    */
   transmux(data) {
+    goog.asserts.assert(this.muxTransmuxer_,
+        'mux.js should be available.');
     goog.asserts.assert(!this.isTransmuxing_,
         'No transmuxing should be in progress.');
     this.isTransmuxing_ = true;