diff --git a/source b/source
index a6e8e7527f5..bf3c373dcd4 100644
--- a/source
+++ b/source
@@ -73638,7 +73638,12 @@ dictionary <dfn dictionary>ElementDefinitionOptions</dfn> {
 
    <dt><code data-x=""><var>registry</var>.<span subdfn data-x="dom-CustomElementRegistry-initialize">initialize</span>(<var>root</var>)</code></dt>
    <dd>Each <span>inclusive descendant</span> of <var>root</var> with a null registry will have it
-   updated to this <code>CustomElementRegistry</code> object.</dd>
+   updated to this <code>CustomElementRegistry</code> object. A
+   <span>"<code>NotSupportedError</code>"</span> <code>DOMException</code> will be thrown if this
+   <code>CustomElementRegistry</code> object is not for scoped usage and <var>root</var> is either a
+   <code>Document</code> node or <var>root</var>'s <span>node document</span>'s <span
+   data-x="document-custom-element-registry">custom element registry</span> is not this
+   <code>CustomElementRegistry</code> object.</dd>
   </dl>
 
   <p w-nodev>The <dfn
@@ -73987,6 +73992,12 @@ console.assert(el instanceof SpiderMan);    // upgraded!</code></pre>
   steps are:</p>
 
   <ol>
+   <li><p>If <span>this</span>'s <span>is scoped</span> is false and <var>root</var> is either a
+   <code>Document</code> node or <var>root</var>'s <span>node document</span>'s <span
+   data-x="document-custom-element-registry">custom element registry</span> is not
+   <span>this</span>, then throw a <span>"<code>NotSupportedError</code>"</span>
+   <code>DOMException</code>.</p></li>
+
    <li><p>If <var>root</var> is a <code>Document</code> node whose <span
    data-x="document-custom-element-registry">custom element registry</span> is null, then set
    <var>root</var>'s <span data-x="document-custom-element-registry">custom element registry</span>