[O] Word "nil" appears after figures in HTML export

[Richard Stanton]

Starting some time in the last few days, when I export an org file to HTML
I find the word "nil" appearing right after the figure. Here's an example
of the HTML code generated:

<div class="figure">
<p><img src="images/RS_head_cropped.png"
alt="RS_head_cropped.png"/></p>nil
</div>

Just to be clear, there's no word "nil" in my org file...