i get an error that kicks you from the server when signs are placed or present on the server with signs on a server

So i was on my friends server which he made a 15w31a server and i tried placing down a sign and i got an error message and was kicked from the server my friend who is the server host also got the same error message and we were both unable to join the server until he changed the world. this is not a single player bug only multiplayer