Crash when opening 13w39b world: java.lang.ClassCastException: mp cannot be cast to ji

Ticking memory connection

java.lang.ClassCastException: mp cannot be cast to ji
	at jj.a(SourceFile:10)
	at el.a(SourceFile:145)
	at mr.c(SourceFile:134)
	at net.minecraft.server.MinecraftServer.u(SourceFile:529)
	at net.minecraft.server.MinecraftServer.t(SourceFile:441)
	at bsb.t(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:358)
	at kx.run(SourceFile:617)

Console:

Client> [21:44:55] [Server thread/INFO]: Starting integrated minecraft server version 13w42a
Client> [21:44:55] [Server thread/INFO]: Generating keypair
Client> [21:44:55] [Server thread/INFO]: Preparing start region for level 0
Client> [21:44:56] [Server thread/ERROR]: Encountered an unexpected exception
Client> s: Ticking memory connection
Client> 	at mr.c(SourceFile:147) ~[13w42a.jar:?]
Client> 	at net.minecraft.server.MinecraftServer.u(SourceFile:529) ~[13w42a.jar:?]
Client> 	at net.minecraft.server.MinecraftServer.t(SourceFile:441) ~[13w42a.jar:?]
Client> 	at bsb.t(SourceFile:117) ~[13w42a.jar:?]
Client> 	at net.minecraft.server.MinecraftServer.run(SourceFile:358) [13w42a.jar:?]
Client> 	at kx.run(SourceFile:617) [13w42a.jar:?]
Client> Caused by: java.lang.ClassCastException: mp cannot be cast to ji
Client> 	at jj.a(SourceFile:10) ~[13w42a.jar:?]
Client> 	at el.a(SourceFile:145) ~[13w42a.jar:?]
Client> 	at mr.c(SourceFile:134) ~[13w42a.jar:?]
Client> 	... 5 more
Client> [21:44:56] [Server thread