diff --git a/index.html b/index.html
index a35c9cd4..cf112528 100644
--- a/index.html
+++ b/index.html
@@ -3391,6 +3391,9 @@ <h2>Contexts</h2>
     the <a>web frame identifier</a> property
     from <var>object</var>.
 
+  <li><p>If <var>reference</var> is not a <a>String</a>,
+    return an <a>error</a> with <a>error code</a> <a>invalid argument</a>.
+
   <li><p>Let <var>browsing context</var> be the <a>browsing context</a> whose
     <a>window handle</a> is <var>reference</var>, or null if no such
     <a>browsing context</a> exists.
@@ -3414,6 +3417,9 @@ <h2>Contexts</h2>
     the <a>web window identifier</a> property
     from <var>object</var>.
 
+  <li><p>If <var>reference</var> is not a <a>String</a>,
+    return an <a>error</a> with <a>error code</a> <a>invalid argument</a>.
+
   <li>
     <p>Let <var>browsing context</var> be the <a>browsing context</a> whose
       <a>window handle</a> is <var>reference</var>, or null if no such
@@ -4475,6 +4481,9 @@ <h2>Elements</h2>
   <a data-lt="getting a property">getting</a> the <a>web element
   identifier</a> property from <var>object</var>.
 
+ <li><p>If <var>reference</var> is not a <a>String</a>,
+  return an <a>error</a> with <a>error code</a> <a>invalid argument</a>.
+
  <li><p>Let <var>element</var> be the result
   of <a>trying</a> to <a>get a known element</a>
   with <var>session</var> and <var>reference</var>.
@@ -4785,6 +4794,9 @@ <h3 id=shadow-root>Shadow Roots</h3>
    the <a>shadow root identifier</a> property
    from <var>object</var>.
 
+  <li><p>If <var>reference</var> is not a <a>String</a>,
+   return an <a>error</a> with <a>error code</a> <a>invalid argument</a>.
+
   <li><p>Let <var>shadow</var> be the result
    of <a>trying</a> to <a>get a known shadow root</a>
    with <var>session</var> and <var>reference</var>.