Derparatron

Hello! I've got a little problem and I don't know how to deal with it. So I've just recently started to fix the server version (works on singleplayer not multiplayer) and I started the server. It started normally but when I go to Mutliplayer on the client and try to join the server, I get: Failed to login: Invalid session (Try restarting your game) I thought I'd see an error in the console but I didn't, all I got was: [16:36:59] [server thread/INFO] [net.minecraft.server.network.NetHandlerLoginServer]: GameProfile{id='null', name='derpplayer'} (/127.0.0.1:1785) lost connection: Disconnected Anyone know what can cause this? Need anything like code or stuff just ask (not sure what to give you)