diff --git a/index.bs b/index.bs
index 3ba62453..75e707c9 100644
--- a/index.bs
+++ b/index.bs
@@ -5372,8 +5372,9 @@ shape for developers. Specifically:
     of the names in the [=error names table=].
 *   The [=interface=] must have a [=constructor operation=] which sets the instance's
     [=DOMException/name=] to the interface's [=identifier=].
-*   Their [=constructor operation=] must take as its first parameter a {{DOMString}} named |message|,
-    and must set the instance's [=DOMException/message=] to |message|.
+*   Their [=constructor operation=] must take as its first parameter an [=optional=] {{DOMString}}
+    named |message| defaulting to the empty string, and must set the instance's
+    [=DOMException/message=] to |message|.
 *   Their [=constructor operation=] should take as its second parameter a [=dictionary=] containing
     the additional information that needs to be exposed.
 *   They should have [=read only=] [=attributes=], whose names are the same as the members of the
@@ -5392,7 +5393,7 @@ interfaces will always return 0.
     <pre highlight=webidl>
         [Exposed=Window]
         interface WidgetError : DOMException {
-          constructor(DOMString message, WidgetErrorOptions options);
+          constructor(optional DOMString message = "", WidgetErrorOptions options);
 
           readonly attribute unsigned long long hardwareErrorCode;
         };