TypeError : App\HashMap\HashMap::normalize(): Return value must be of type array|string|int|float|bool, ArrayObject returned

<?php

namespace App\HashMap;

use Symfony\Component\Serializer\Normalizer\DenormalizableInterface;
use Symfony\Component\Serializer\Normalizer\DenormalizerInterface;
use Symfony\Component\Serializer\Normalizer\NormalizableInterface;
use Symfony\Component\Serializer\Normalizer\NormalizerInterface;

class HashMap implements NormalizableInterface, DenormalizableInterface
{
    private \ArrayObject $items;

    public function __construct(array $items = [])
    {
        $this->items = new \ArrayObject($items);
    }

    public function add(string $key, string $value): self {
        $this->items->offsetSet($key, $value);

        return $this;
    }

    public function has(string $key): bool {
        return $this->items->offsetExists($key);
    }

    public function remove(string $key): void {
        $this->items->offsetUnset($key);
    }

    public function denormalize(
        DenormalizerInterface       $denormalizer,
        float|int|bool|array|string $data,
        string                      $format = null,
        array                       $context = []
    ) {
        return new self();
    }

    public function normalize(
        NormalizerInterface $normalizer,
        string              $format = null,
        array               $context = []
    ): array|string|int|float|bool {
        return $this->items;
    }
}

<?php

/*
 * This file is part of the Symfony package.
 *
 * (c) Fabien Potencier <fabien@symfony.com>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

namespace Symfony\Component\Serializer\Normalizer;

/**
 * Defines the most basic interface a class must implement to be normalizable.
 *
 * If a normalizer is registered for the class and it doesn't implement
 * the Normalizable interfaces, the normalizer will be used instead.
 *
 * @author Jordi Boggiano <j.boggiano@seld.be>
 */
interface NormalizableInterface
{
    /**
     * Normalizes the object into an array of scalars|arrays.
     *
     * It is important to understand that the normalize() call should normalize
     * recursively all child objects of the implementor.
     *
     * @param NormalizerInterface $normalizer The normalizer is given so that you
     *                                        can use it to normalize objects contained within this object
     * @param string|null         $format     The format is optionally given to be able to normalize differently
     *                                        based on different output formats
     * @param array               $context    Options for normalizing this object
     */
    public function normalize(NormalizerInterface $normalizer, string $format = null, array $context = []): array|string|int|float|bool|\ArrayObject|null;
}