What causes "This store does not support null keys" in hibernate/hibernate-orm?

Check key != null before put and handle the missing-entity case explicitly

hibernate/hibernate-orm · error · NullPointerException

This store does not support null keys

Error message

This store does not support null keys

What it means

InstanceIdentityStore.put(Object key, int instanceId, V value) rejects null keys up front with NullPointerException and 'This store does not support null keys'. Entries are addressed by instanceId, but the key is stored alongside for identity verification (the k == key checks in get/remove), so a null key cannot be stored without breaking that verification.

Source

Thrown at hibernate-core/src/main/java/org/hibernate/internal/util/collections/InstanceIdentityStore.java:81

				throw new ConcurrentModificationException(
						"Found a different instance corresponding to instanceId [" + instanceId +
						"], this might indicate a concurrent access to this persistence context."
				);
			}
		}
		return null;
	}

	/**
	 * Associates the specified value with the specified key in this store (optional operation). If the store
	 * previously contained a mapping for the key, the old value is replaced by the specified value.
	 *
	 * @param key key with which the specified value is to be associated
	 * @param value value to be associated with the specified key
	 */
	public void put(Object key, int instanceId, V value) {
		if ( key == null ) {
			throw new NullPointerException( "This store does not support null keys" );
		}
		else if ( instanceId <= 0 ) {
			throw new IllegalArgumentException( "Instance ID must be a positive value" );
		}

		final int keyIndex = toKeyIndex( instanceId );
		final Page<Object> page = getOrCreateEntryPage( keyIndex );
		final int pageOffset = toPageOffset( keyIndex );
		page.set( pageOffset, key );
		page.set( pageOffset + 1, value );
	}

	/**
	 * Removes the mapping for an instance id from this store if it is present (optional operation).
	 *
	 * @param instanceId the instance id whose associated value is to be returned
	 * @param key key instance to double-check instance equality
	 * @implNote This method accesses the backing array with the provided instance id, but performs an instance

View on GitHub (pinned to fad1729dce)

Solutions

Check key != null before put and handle the missing-entity case explicitly
Trace the null to its source — usually session.find/get on a deleted or never-existing row
Do not 'fix' by passing a placeholder key; the identity checks would then throw elsewhere

Example fix

// before
store.put( entity, id, state );
// after
if ( entity == null ) {
    return; // entity absent: nothing to cache
}
store.put( entity, id, state );

Defensive patterns

Strategy: validation

Validate before calling

Objects.requireNonNull( key, "entity key must not be null" );
store.put( key, instanceId, value );

Prevention

Check entity references for null right after load/find, before caching them
Model 'entity not found' explicitly instead of storing null keys
Fail fast with a descriptive message at your own API boundary rather than letting the store throw

When it happens

Trigger: put(null, instanceId, value) — an entity reference that is null because a preceding load/find returned null (row absent) or a variable was never assigned.

Common situations: Cache-fill code that puts the result of session.get() without a null check for missing rows; refactoring that turned a guaranteed non-null key into an Optional-unwrapped null.

Related errors

AI-assisted analysis of hibernate/hibernate-orm@fad1729dce (2026-08-22). Data as JSON: /api/errors/e72d3895974608d8. Report an issue: GitHub.