Multiplayer server crashes when the host joins

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here

What actually happened was...:
Describe what happened here

Steps to Reproduce:
1. Start hosting server
2. Join server
3. It will then crash

Description: Watching Server

java.lang.Error
	at java.util.TreeMap.successor(Unknown Source)
	at java.util.TreeMap$PrivateEntryIterator.nextEntry(Unknown Source)
	at java.util.TreeMap$KeyIterator.next(Unknown Source)
	at ql.a(SourceFile:566)
	at ql.a(SourceFile:546)
	at bet.a(SourceFile:258)
	at bet.a(SourceFile:113)
	at qk.b(SourceFile:164)
	at qk.d(SourceFile:235)
	at ql.c(SourceFile:185)
	at net.minecraft.server.MinecraftServer.y(SourceFile:597)
	at ph.y(SourceFile:303)
	at net.minecraft.server.MinecraftServer.x(SourceFile:529)
	at net.minecraft.server.MinecraftServer.run(SourceFile:445)
	at java.lang.Thread.run(Unknown Source)