diff --git a/types/wicg-mediasession/index.d.ts b/types/wicg-mediasession/index.d.ts
index ac08c8ef4f..05a769b2a2 100644
--- a/types/wicg-mediasession/index.d.ts
+++ b/types/wicg-mediasession/index.d.ts
@@ -19,10 +19,10 @@ interface MediaSession {
   // Current media session playback state.
   playbackState: MediaSessionPlaybackState;
   // Current media session meta data.
-  metadata?: MediaMetadata|null;
+  metadata: MediaMetadata|null;
 
   // Set/Unset actions handlers.
-  setActionHandler(action: MediaSessionAction, listener?: () => void): void;
+  setActionHandler(action: MediaSessionAction, listener: (() => void)|null): void;
 }
 
 interface MediaImage {