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[1.19.2] Cannot invoke "net.minecraft.network.syncher.SynchedEntityData$DataItem.getValue()" because "dataitem" is null


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Error when trying to summon entity:

[12:09:12] [Server thread/WARN] [minecraft/EntityType]: Exception loading entity: 
java.lang.NullPointerException: Cannot invoke "net.minecraft.network.syncher.SynchedEntityData$DataItem.getValue()" because "dataitem" is null

I followed these docs to create what I have below in my entity class:

public abstract class Dinosaur extends Animal {
    static final EntityDataAccessor<Integer> serum = SynchedEntityData.defineId(Dinosaur.class, EntityDataSerializers.INT);
    protected Dinosaur(EntityType<? extends Animal> e, Level l) {
        super(e, l);
    public boolean isJuvenile() {
        return this.isBaby() && this.getSerum() > 0;
    public void tick() {
        if (this.getSerum() >= 4) {
            this.age = 0;
            if (this.isBaby()) {
        else {
            if (!this.isBaby()) {
            this.age = -20;
    public void defineSynchedData() {
        this.entityData.define(serum, 1);
    public void addSerum() {
        int s = this.getSerum();
        this.getEntityData().set(serum, s++);
    public int getSerum() {
        return this.getEntityData().get(serum);


Any help is greatly appreciated, thank you :)

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