Sony Xperia Tablet Z can't setup proxy

When I'm on my tablet, I go to my wifi network, tap and hold on it, and I get a popup menu. I hit modify network, and get a popup for the wifi. I tap "show advanced options", and then change Proxy Settings from None -> Manual. The moment I do this the Save button grays out, and I can't click it. Even if I fill in the proxy hostname and port, the save button remains grayed out. Do I need root? Sorry if this is fairly obvious, I just found nothing online.