reading list add page is greyed out

I am on a site where I have just saved/added a page to my reading list, and I have just gone to another link on the site and wanted to save it as well, but the ADD PAGE button is greyed out and won't allow me to save it to the Reading List.

The site is GIZMODO, what gives????

I am using MAC OS X 10.8 and Safari 6.0 on a 15" Macbook Pro 2010.