code: public function validate()
  	{
  		$varValue = $this->getPost($this->strName);
  
  		if (!empty($varValue) && !$this->isValidOption($varValue))
  		{
  			$this->addError($GLOBALS['TL_LANG']['ERR']['invalid']);
  		}
  
  		parent::validate();
  	}
docs: Check for a valid option (see #4383)