diff --git a/lib/polyfill/media_capabilities.js b/lib/polyfill/media_capabilities.js
index 2b758b5f28..4bb9073d77 100644
--- a/lib/polyfill/media_capabilities.js
+++ b/lib/polyfill/media_capabilities.js
@@ -203,8 +203,10 @@ shaka.polyfill.MediaCapabilities = class {
 
       const cacheKey = shaka.polyfill.MediaCapabilities
           .generateKeySystemCacheKey_(
-              mediaDecodingConfig.video.contentType,
-              mediaDecodingConfig.audio.contentType,
+              mediaDecodingConfig.video ?
+                  mediaDecodingConfig.video.contentType : '',
+              mediaDecodingConfig.audio ?
+                  mediaDecodingConfig.audio.contentType : '',
               mediaDecodingConfig.keySystemConfiguration.keySystem);
 
       let keySystemAccess;