diff --git a/source b/source
index 90c6259281b..df8ca1ea533 100644
--- a/source
+++ b/source
@@ -57400,8 +57400,8 @@ interface <dfn interface>HTMLSelectElement</dfn> : <span>HTMLElement</span> {
   <ol>
    <li><p>If <var>event</var>'s <span>canceled flag</span> is set, then return.</p></li>
 
-   <li><p>If <var>event</var>'s <code data-x="dom-MouseEvent-button">button</code> attribute is not
-   1, then return.</p></li>
+   <li><p>If <var>event</var> is a <code>MouseEvent</code> and <var>event</var>'s <code
+   data-x="dom-MouseEvent-button">button</code> attribute is not 1, then return.</p></li>
 
    <li><p>Run the <span>show popover</span> algorithm given <var>select</var>'s <span>select
    popover</span>, false, and <var>select</var>.</p></li>