Your feed seems to be fine: http://feeds.feedburner.com/multijokebookfeed
feedvalidator.org is outdated, and your feed validates at http://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Ffeeds.feedburner.com%2Fmultijokebookfeed but it looks like whatever you're using to minify your files will cause trouble for lots of feed readers.
