“NOT AUTHENTICATED” warning trying to install rdesktop on almost-fresh Mint 16 install

Linux Mint 16 (Ubuntu-based) 32-bit running on a netbook. Installed Wine and Skype, and all updates. Went to install rdesktop (typed rdesktop in the gnome menu search, chose 'install rdesktop'), and I get a screen similar to , except that under the heading "NOT AUTHENTICATED," when I click the plus, there's just one blank line, no actual package names. The summary says 10 will be held back, 1 will be installed. I assume the 1 is rdesktop, so what's this blank line and warning all about? Will I fall victim to some MitM attack if I click "apply"?