vB forums RSS feeds missing element

[محروم.كوم]

My Google webmaster tools tells me this about my vb forums RSS feeds:
Quote:
Missing thumbnail

We've detected that one of the URLS in your Sitemap contains a missing or invalid reference to a thumbnail image file. Please ensure that each URL contains element specifying a valid image URL.
Am I doing something wrong? All my forum RSS feeds have this error. __DEFINE_LIKE_SHARE__