HTML doesn't work in Upfront?

I have the following html in a "code" element in a Panino install:

<div id="wrapper">
    <img src="http://placehold.it/300x200" class="hover" />
    <p class="text">text</p>
</div>

The site is giving me an error saying the html is unacceptable and needs to be checked for errors. I must be going crazy. What am I missing? I can't imagine html being more simple than this. Am I trying to do something that Upfront doesn't accommodate?