Issue with Drowned Mob causing crashes.

Random crashes, log says 'ticking entity' and mentions 'Drowned' mob.

Tried attaching the report, but the forum gives me an error so here it is below:

Minecraft Crash Report ----
// Would you like a cupcake?

Time: 3/16/18 11:41 PM
Description: Ticking entity

java.lang.ClassCastException: afn cannot be cast to afj
at adb.a(SourceFile:41)
at acz.a(SourceFile:22)
at adl.a(SourceFile:94)
at abw.cL(SourceFile:644)
at abv.l(SourceFile:1895)
at abw.l(SourceFile:504)
at ajs.l(SourceFile:33)
at aki.l(SourceFile:257)
at abv.S_(SourceFile:1750)
at abw.S_(SourceFile:284)
at ajs.S_(SourceFile:38)
at aki.S_(SourceFile:232)
at ajg.S_(SourceFile:252)
at aug.a(SourceFile:1617)
at se.a(SourceFile:705)
at aug.h(SourceFile:1593)
at aug.p_(SourceFile:1475)
at se.p_(SourceFile:538)
at net.minecraft.server.MinecraftServer.w(SourceFile:702)
at net.minecraft.server.MinecraftServer.v(SourceFile:627)
at czd.v(SourceFile:155)
at net.minecraft.server.MinecraftServer.run(SourceFile:532)
at java.lang.Thread.run(Thread.java:745)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– Head –
Thread: Server thread
Stacktrace:
at adb.a(SourceFile:41)
at acz.a(SourceFile:22)
at adl.a(SourceFile:94)
at abw.cL(SourceFile:644)
at abv.l(SourceFile:1895)
at abw.l(SourceFile:504)
at ajs.l(SourceFile:33)
at aki.l(SourceFile:257)
at abv.S_(SourceFile:1750)
at abw.S_(SourceFile:284)
at ajs.S_(SourceFile:38)
at aki.S_(SourceFile:232)
at ajg.S_(SourceFile:252)
at aug.a(SourceFile:1617)
at se.a(SourceFile:705)
at aug.h(SourceFile:1593)

– Entity being ticked –
Details:
Entity Type: minecraft:drowned (ajg)
Entity ID: 244
Entity Name: Drowned
Entity's Exact location: -614.30, 40.27, -83.81
Entity's Block location: World: (-615,40,-84), Chunk: (at 9,2,12 in -39,-6; contains blocks -624,0,-96 to -609,255,-81), Region: (-2,-1; contains chunks -64,-32 to -33,-1, blocks -1024,0,-512 to -513,255,-1)
Entity's Momentum: 0.00, 0.07, 0.00
Entity's Passengers: []
Entity's Vehicle: ~_ERROR~ NullPointerException: null
Stacktrace:
at aug.p_(SourceFile:1475)
at se.p_(SourceFile:538)

– Affected level –
Details:
Level name: Snapshot w8a09?
All players: 1 total; [sg['BillyOatmeal'/20, l='Snapshot w8a09?', x=-577.43, y=11.00, z=-128.02]]
Chunk stats: ServerChunkCache: 881 Drop: 0
Level seed: -940041908687425
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: World: (-228,64,-84), Chunk: (at 12,4,12 in -15,-6; contains blocks -240,0,-96 to -225,255,-81), Region: (-1,-1; contains chunks -32,-32 to -1,-1, blocks -512,0,-512 to -1,255,-1)
Level time: 1577013 game time, 2125909 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 152407 (now: false), thunder time: 18135 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
at net.minecraft.server.MinecraftServer.w(SourceFile:702)
at net.minecraft.server.MinecraftServer.v(SourceFile:627)
at czd.v(SourceFile:155)
at net.minecraft.server.MinecraftServer.run(SourceFile:532)
at java.lang.Thread.run(Thread.java:745)

– System Details –
Details:
Minecraft Version: 18w11a
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 331190128 bytes (315 MB) / 603979776 bytes (576 MB) up to 1073741824 bytes (1024 MB)
JVM Flags: 8 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [sg['BillyOatmeal'/20, l='Snapshot w8a09?', x=-577.43, y=11.00, z=-128.02]]
Data Packs: vanilla
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.