Why does the error "This value should be of type array." occur?

Question

symfony

The form has a "categories" field:

->add('categories', EntityType::class, [
    'choices' => $categories,
    'choice_label' => function (Category $category = null) {
        return null !== $category ? str_repeat('--', $category->getLevel()).' '.$category->getTitle() : '';
    },
    'choice_value' => function (Category $category = null) {
        return null !== $category ? $category->getId() : '';
    },
    'class' => Category::class,
    'constraints' => [
        new Assert\Choice([
            'choices' => $categories,
            'multipleMessage' => 'Выберите разделы из предложенного списка.',
            'multiple' => true,
        ]),
    ],
    'label' => 'Разделы',
    'multiple' => true,
 ])

After submitting the form for the field "categories" I get the error "This value should be of type array.". In the controller I check the type:

if ($form->isSubmitted()) {
    echo gettype($_POST['categories']); // Выводит "array"
}

What could be the reason?
Thank you all in advance.