Collapsing Page Menu plugin not validating

I am trying to eliminate any validation errors with my blog

But the plugin produces:

<ul>

    <li>List item 1</li>
    <li>List item 2</li>

        <ul>

            <li>Sub list item 2a</li>
            <li>Sub list item 2b</li> 

        </ul> 

    <li>List item 3</li> 

</ul>

In other words a sublist should sit fully inside the
<li> tag.

I’ve had a quick look at the php code but am completely lost.

Has anyone had a look at this before or are able to point me in the right direction?

Thanks for your help 🙂