[MC-89894] Mobs riding slimes cause the game to crash

If you summon a slime that is being ridden by another mob using commands, a crash will eventually occur.

This is my crash report:

---- Minecraft Crash Report ----
// You're mean.

Time: 7/10/15 8:41 AM
Description: Ticking entity

java.lang.ClassCastException: yq cannot be cast to rs
	at axt.a(SourceFile:31)
	at uo.a(SourceFile:172)
	at uo.a(SourceFile:97)
	at up.k(SourceFile:236)
	at up.j(SourceFile:168)
	at rm.bV(SourceFile:560)
	at rl.n(SourceFile:1588)
	at rm.n(SourceFile:409)
	at rl.m(SourceFile:1456)
	at rm.m(SourceFile:240)
	at yq.m(SourceFile:117)
	at agw.a(SourceFile:1462)
	at lk.a(SourceFile:705)
	at agw.g(SourceFile:1440)
	at agw.k(SourceFile:1330)
	at lk.k(SourceFile:538)
	at net.minecraft.server.MinecraftServer.D(SourceFile:618)
	at net.minecraft.server.MinecraftServer.C(SourceFile:546)
	at bvr.C(SourceFile:154)
	at net.minecraft.server.MinecraftServer.run(SourceFile:450)
	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 --
Stacktrace:
	at axt.a(SourceFile:31)
	at uo.a(SourceFile:172)
	at uo.a(SourceFile:97)
	at up.k(SourceFile:236)
	at up.j(SourceFile:168)
	at rm.bV(SourceFile:560)
	at rl.n(SourceFile:1588)
	at rm.n(SourceFile:409)
	at rl.m(SourceFile:1456)
	at rm.m(SourceFile:240)
	at yq.m(SourceFile:117)
	at agw.a(SourceFile:1462)
	at lk.a(SourceFile:705)
	at agw.g(SourceFile:1440)

-- Entity being ticked --
Details:
	Entity Type: Slime (yq)
	Entity ID: 786
	Entity Name: Slime
	Entity's Exact location: -233.81, 64.00, 228.24
	Entity's Block location: World: (-234,64,228), Chunk: (at 6,4,4 in -15,14; contains blocks -240,0,224 to -225,255,239), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
	Entity's Momentum: 0.04, -0.08, -0.06
	Entity's Rider: yc['Creeper'/785, l='Slime Crash World', x=-233.81, y=65.53, z=228.24]
	Entity's Vehicle: ~~ERROR~~ NullPointerException: null
Stacktrace:
	at agw.k(SourceFile:1330)
	at lk.k(SourceFile:538)

-- Affected level --
Details:
	Level name: Slime Crash World
	All players: 1 total; [lm['Cavinator1'/644, l='Slime Crash World', x=-234.06, y=66.17, z=232.23]]
	Chunk stats: ServerChunkCache: 272 Drop: 0
	Level seed: -2266167899765054753
	Level generator: ID 00 - default, ver 1. Features enabled: true
	Level generator options: 
	Level spawn location: World: (-242,64,232), Chunk: (at 14,4,8 in -16,14; contains blocks -256,0,224 to -241,255,239), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
	Level time: 1677 game time, 14000 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 48971 (now: false), thunder time: 170183 (now: false)
	Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
	at net.minecraft.server.MinecraftServer.D(SourceFile:618)
	at net.minecraft.server.MinecraftServer.C(SourceFile:546)
	at bvr.C(SourceFile:154)
	at net.minecraft.server.MinecraftServer.run(SourceFile:450)
	at java.lang.Thread.run(Thread.java:745)

-- System Details --
Details:
	Minecraft Version: 15w40b
	Operating System: Windows 7 (amd64) version 6.1
	Java Version: 1.8.0_25, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 166171328 bytes (158 MB) / 340983808 bytes (325 MB) up to 1060372480 bytes (1011 MB)
	JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
	IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
	Profiler Position: N/A (disabled)
	Player Count: 1 / 8; [lm['Cavinator1'/644, l='Slime Crash World', x=-234.06, y=66.17, z=232.23]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

In the "Entity being ticked" part it says an error under "Entity vehicle."

Steps to Reproduce:
1. Use the following commmand to summon a Creeper riding a Slime:
/summon Creeper ~ ~ ~ {Riding:{id:Slime,Size:3}}
2. Eventually the game will crash.

Notes: I have only tested zombies and creepers riding slimes. Other mobs being ridden on (Like a zombie on a skeleton) seem to work fine. I have not tested slimes of smaller or larger sizes or magma cubes.