diff --git a/index.html b/index.html
index d76a5f3..45f5628 100644
--- a/index.html
+++ b/index.html
@@ -306,7 +306,7 @@ <h2>Extensibility</h2>
   <pre class="idl">
     partial interface Element {
       [CEReactions] attribute [LegacyNullToEmptyString] DOMString outerHTML;
-      [CEReactions] void insertAdjacentHTML(DOMString position, DOMString text);
+      [CEReactions] undefined insertAdjacentHTML(DOMString position, DOMString text);
     };
   </pre>