public function validateSections(ElementInterface $element)
	{
		$value = $element->getFieldValue($this->handle);

		if (!is_array($value)) {
			$value = [$value];
		}

		$sections = $this->getSections();

		foreach ($value as $section) {
			if (!isset($sections[$section])) {
				$element->addError($this->handle, Craft::t('section-field', 'Invalid section selected.'));
			}
		}
	}