Validation errors for attribute "rel" on element "a"

Hello,

I have been working with the Nu Html Checker (https://validator.w3.org/nu/) to validate my new website and there are several erros, that seem to be cause by the Field Slideshow module.

Error: Bad value field-slideshow[nid-1354]image for attribute rel on element a: The string field-slideshow[nid-1354]image is not a registered keyword.

From line 349, column 9; to line 349, column 355

>↩        <a href="http://www.example.com/sites/default/files/screenshots/screenshot1…lideshow[nid-1354]image" title="Some title"><img c

Syntax of list of link-type keywords:
    A whitespace-separated list of link types, with no duplicate keywords in the list. Each link type must be listed as allowed on <a> and <area> in the HTML specification, or must be listed as allowed on <a> and <area> on the Microformats wiki, or must be an absolute URL. You can register link types on the Microformats wiki yourself. 

In case the whole code part is of importance, here is it, copied form the output source of the page:

<div class="field-slideshow-slide field-slideshow-slide-1 even first">
	<a href="http://example.com/sites/default/files/screenshots/screenshot1.jpg?slideshow=true&amp;slideshowAuto=true&amp;slideshowSpeed=8000&amp;speed=350&amp;transition=elastic" class="colorbox-load" rel="field-slideshow[nid-1354]image" title="Some title"><img class="field-slideshow-image field-slideshow-image-1" typeof="foaf:Image" src="http://www.example.com/sites/default/files/styles/screenshot_slideshow/public/screenshots/screenshot1.jpg?itok=HGLYL1RO" width="640" height="463" alt="Some text" title="Some title" /></a>
	<div class="field-slideshow-caption">
		<span class="field-slideshow-caption-text">Some text</span>
	</div>
</div>

Can you please take a look into it?