"Join server" button not clickable on any servers on my server list

When I select a server from my server list, the "Join Server" button doesn't turn the lighter gray colour it should, and therefore is not clickable. This happens to every server on my server list and even restarting minecraft doesn't fix it. And no, it's not just because the servers are 1.5.2. I tested it with a 1.6.1 server and the same issue persisted.