Server crashes everytime

I shot my friend with a bow with Power 4, Flame 1 and Punch 1 enchantments with a spectral arrow.
When I kill directly with the arrow, the server crashes with the following exception:

[15:45:03] [Server thread/ERROR]: Encountered an unexpected exception
f: Ticking entity
        at net.minecraft.server.MinecraftServer.w(SourceFile:683) ~[minecraft_server.18w01a.jar:?]
        at qy.w(SourceFile:352) ~[minecraft_server.18w01a.jar:?]
        at net.minecraft.server.MinecraftServer.v(SourceFile:604) ~[minecraft_server.18w01a.jar:?]
        at net.minecraft.server.MinecraftServer.run(SourceFile:509) [minecraft_server.18w01a.jar:?]
        at java.lang.Thread.run(Thread.java:745) [?:1.8.0_91]
Caused by: java.lang.NullPointerException
        at um.a(SourceFile:28) ~[minecraft_server.18w01a.jar:?]
        at um.a(SourceFile:22) ~[minecraft_server.18w01a.jar:?]
        at um.<init>(SourceFile:15) ~[minecraft_server.18w01a.jar:?]
        at uo.a(SourceFile:24) ~[minecraft_server.18w01a.jar:?]
        at java.util.Map.computeIfAbsent(Map.java:957) ~[?:1.8.0_91]
        at uo.a(SourceFile:24) ~[minecraft_server.18w01a.jar:?]
        at uo.b(SourceFile:41) ~[minecraft_server.18w01a.jar:?]
        at rp.a(SourceFile:456) ~[minecraft_server.18w01a.jar:?]
        at aam.a(SourceFile:849) ~[minecraft_server.18w01a.jar:?]
        at aiz.a(SourceFile:883) ~[minecraft_server.18w01a.jar:?]
        at rp.a(SourceFile:526) ~[minecraft_server.18w01a.jar:?]
        at ajd.a(SourceFile:344) ~[minecraft_server.18w01a.jar:?]
        at ajd.H_(SourceFile:265) ~[minecraft_server.18w01a.jar:?]
        at ajq.H_(SourceFile:31) ~[minecraft_server.18w01a.jar:?]
        at arv.a(SourceFile:1564) ~[minecraft_server.18w01a.jar:?]
        at rn.a(SourceFile:704) ~[minecraft_server.18w01a.jar:?]
        at arv.h(SourceFile:1540) ~[minecraft_server.18w01a.jar:?]
        at arv.l(SourceFile:1422) ~[minecraft_server.18w01a.jar:?]
        at rn.l(SourceFile:537) ~[minecraft_server.18w01a.jar:?]
        at net.minecraft.server.MinecraftServer.w(SourceFile:679) ~[minecraft_server.18w01a.jar:?]
        ... 4 more