A modded server can crash clients with ArrayIndexOutOfBoundsException

java.lang.ArrayIndexOutOfBoundsException: Index 21 out of bounds for length 21
at net.minecraft.network.syncher.SynchedEntityData.assignValues(List)(net/minecraft/network/syncher/SynchedEntityData.java:116)
at net.minecraft.client.multiplayer.ClientPacketListener.handleSetEntityData(ClientboundSetEntityDataPacket)(net/minecraft/client/multiplayer/ClientPacketListener.java:538)
at net.minecraft.network.protocol.game.ClientboundSetEntityDataPacket.handle(ClientGamePacketListener)(net/minecraft/network/protocol/game/ClientboundSetEntityDataPacket.java:54)
at net.minecraft.network.protocol.game.ClientboundSetEntityDataPacket.handle(PacketListener)(net/minecraft/network/protocol/game/ClientboundSetEntityDataPacket.java:12)
at net.minecraft.client.multiplayer.ClientPacketListener.handleBundlePacket(ClientboundBundlePacket)(net/minecraft/client/multiplayer/ClientPacketListener.java:2155)
at net.minecraft.network.protocol.game.ClientboundBundlePacket.handle(ClientGamePacketListener)(net/minecraft/network/protocol/game/ClientboundBundlePacket.java:19)
at net.minecraft.network.protocol.game.ClientboundBundlePacket.handle(PacketListener)(net/minecraft/network/protocol/game/ClientboundBundlePacket.java:7)
at net.minecraft.network.protocol.PacketUtils.ensureRunningOnSameThread(Packet,PacketListener,BlockableEventLoop)(net/minecraft/network/protocol/PacketUtils.java:24)
at net.minecraft.util.thread.BlockableEventLoop.doRunTask(Runnable)(net/minecraft/util/thread/BlockableEventLoop.java:162)
at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(Runnable)(net/minecraft/util/thread/ReentrantBlockableEventLoop.java:23)
at net.minecraft.util.thread.BlockableEventLoop.pollTask()(net/minecraft/util/thread/BlockableEventLoop.java:136)
at net.minecraft.util.thread.BlockableEventLoop.runAllTasks()(net/minecraft/util/thread/BlockableEventLoop.java:121)
at net.minecraft.client.Minecraft.runTick(boolean)(net/minecraft/client/Minecraft.java:1285)
at net.minecraft.client.Minecraft.run()(net/minecraft/client/Minecraft.java:889)
at net.minecraft.client.main.Main.main(String[])(net/minecraft/client/main/Main.java:265)