The title basically says it all. I have the following field definition in an Entity class:

$fields['code'] = BaseFieldDefinition::create('integer')
        ->setLabel(t('unique code'))
        ->setDescription(t('Some unique code'))
        ->addConstraint('UniqueField')
        ->setTranslatable(false)
        ->setRequired(true)
        ->setDisplayOptions('view', array(
            'label' => 'above',
            'weight' => 4,
        ))
        ->setDisplayOptions('form', array(
            'weight' => 4,
        ))
        ->setDisplayConfigurable('form', true)
        ->setDisplayConfigurable('view', true);

Creating a new entity in the UI works as expected as long as I do not set the code value to something already in the database. However, as soon as I try to create a new entity with an already existing value for code (thus triggering a constraint violation), I get the following error:

The website encountered an unexpected error. Please try again later.
TypeError: Argument 1 passed to DrupalCoreFormFormState::setError() must be of the type array, null given, called in /var/www/html/web/core/lib/Drupal/Core/Field/WidgetBase.php on line 454 in DrupalCoreFormFormState->setError() (line 1156 of core/lib/Drupal/Core/Form/FormState.php).

If I change the field definition to use a string instead everything works as expected:

$fields['code'] = BaseFieldDefinition::create('string')
        ->setLabel(t('unique code'))
        ->setDescription(t('Some unique code'))
        ->setSetting('max_length', 5)
        ->addConstraint('UniqueField')
        ->setTranslatable(false)
        ->setRequired(true)
        ->setDisplayOptions('view', array(
            'label' => 'above',
            'weight' => 4,
        ))
        ->setDisplayOptions('form', array(
            'weight' => 4,
        ))
        ->setDisplayConfigurable('form', true)
        ->setDisplayConfigurable('view', true);

So the question is how do I add a unique constraint for an integer field? Is there another validation class I can use, am I setting up my integer field definition wrong thus causing the error, or?