Server console regularly reporting 2 errors

Console reports the following two errors with alarming frequency on startup:
[Server thread/WARN]: Unable to resolve BlockEntity for ItemInstance: minecraft:shield

and

java.lang.Error: class cj
at ll.a(SourceFile:479) [minecraft_server.15w36c.jar:?]
at ll.a(SourceFile:472) [minecraft_server.15w36c.jar:?]
at aii.a(SourceFile:141) [minecraft_server.15w36c.jar:?]
at aii.a(SourceFile:118) [minecraft_server.15w36c.jar:?]
at rb.T(SourceFile:778) [minecraft_server.15w36c.jar:?]
at rb.d(SourceFile:721) [minecraft_server.15w36c.jar:?]
at lu.a(SourceFile:350) [minecraft_server.15w36c.jar:?]
at iq.a(SourceFile:126) [minecraft_server.15w36c.jar:?]
at iq$a.a(SourceFile:57) [minecraft_server.15w36c.jar:?]
at fh$1.run(SourceFile:13) [minecraft_server.15w36c.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471) [?:1.7.0_75]
at java.util.concurrent.FutureTask.run(FutureTask.java:262) [?:1.7.0_75]
at g.a(SourceFile:45) [minecraft_server.15w36c.jar:?]
at net.minecraft.server.MinecraftServer.D(SourceFile:597) [minecraft_server.15w36c.jar:?]
at kv.D(SourceFile:314) [minecraft_server.15w36c.jar:?]
at net.minecraft.server.MinecraftServer.C(SourceFile:553) [minecraft_server.15w36c.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:457) [minecraft_server.15w36c.jar:?]
at java.lang.Thread.run(Thread.java:745) [?:1.7.0_75]