Server crash: NullPointerException in Entity.startRiding() during Zombie.spawn (Paper 1.21.10-70) #13182
Description
Expected behavior
Zombies (and other mobs) should spawn normally without causing the server to crash, even when natural spawning or jockey mechanics are involved.
Observed/Actual behavior
The server crashed while ticking the overworld.
The crash report indicates that a Zombie entity attempted to ride another entity (startRiding()), but its vehicle field was null, causing a NullPointerException. No plugins on the server modify riding behavior or mob spawning.
Crash log snippet:
Caused by: java.lang.NullPointerException: Cannot read field "type" because "this.vehicle" is null
at net.minecraft.world.entity.Entity.startRiding(Entity.java:3235)
at net.minecraft.world.entity.Mob.startRiding(Mob.java:1451)
at net.minecraft.world.entity.monster.Zombie.finalizeSpawn(Zombie.java:538)
Steps/models to reproduce
Reproduction is currently uncertain, but the crash occurred during normal gameplay within minutes after restaring and rejoining the server after previous crashes
We have like 2 natural zombie spawners right next to spawn underground
No special configuration, no custom entities, and no mods/plugins affecting entity behavior.
Plugin and Datapack List
No plugins installed - vanilla Paper server with default configuration.
Datapacks: [Vanilla]
Paper version
This server is running Paper version 1.21.10-70-ffcb7b2 (2024-10-13T10:36:24Z) (Implementing API version 1.21.10-R0.1-SNAPSHOT)
You are running the latest version
Previous version: 1.21.1-133-c83d945 (MC: 1.21.1)
Other
Stack trace from crash:
net.minecraft.ReportedException: Exception ticking world
at net.minecraft.server.MinecraftServer.tickChildren(MinecraftServer.java:1799)
[... full stack trace as provided ...]
Caused by: java.lang.NullPointerException: Cannot read field "type" because "this.vehicle" is null
at net.minecraft.world.entity.Entity.startRiding(Entity.java:3235)
at net.minecraft.world.entity.Mob.startRiding(Mob.java:1451)
at net.minecraft.world.entity.monster.Zombie.finalizeSpawn(Zombie.java:538)
This crash occurred multiple times