Why is null value not written, while nullable=true in essence?

Question

symfony

Tell me what could be the problem: there is a Section entity, it has the following code:

/**
     * @ORM\Column(type="string", length=255, nullable=true)
     */
    private $sending_type;

in the shape of:

->add('sending_type', \Symfony\Component\Form\Extension\Core\Type\IntegerType::class, [
                'required' => false
            ])

And the error when submitting the form:

Expected argument of type "string", "null" given at property path "sending_type".