core-bundle/src/Resources/contao/widgets/CheckBoxWizard.php
public function validate()
	{
		$varValue = $this->getPost($this->strName);

		if (!empty($varValue) && !$this->isValidOption($varValue))
		{
			$this->addError($GLOBALS['TL_LANG']['ERR']['invalid']);
		}

		parent::validate();
	}
public function validate()
	{
		$varValue = $this->getPost($this->strName);

		if (!empty($varValue) && !$this->isValidOption($varValue))
		{
			$this->addError($GLOBALS['TL_LANG']['ERR']['invalid']);
		}

		parent::validate();
	}
Check for a valid option (see #4383)