diff --git a/CaptionDisplaySettings/README.md b/CaptionDisplaySettings/README.md
index 937d805..6697070 100644
--- a/CaptionDisplaySettings/README.md
+++ b/CaptionDisplaySettings/README.md
@@ -48,8 +48,8 @@ programming on their screen while changing the closed captioning display setting
 
 ```idl
 dictionary CaptionDisplaySettingsOptions {
-	attribute Node? anchorNode;
-	attribute CSSOMString? positionArea;
+	attribute Node? anchorNode = null;
+	attribute CSSOMString? positionArea = null;
 };
 
 partial interface HTMLMediaElement {