Zend raises an exception if my email subject contains non-ASCII charcters. How to solve it please?

EmailService.php

/*
* Here is one method of my email service.
* When the email subject contains non ASCII characters, its raised and exeption "invalid subject detected"
* It works like a charm if subject only contains ASCII characters
*/
	
	/**
     * Send an email
     * @param \xxxxx\Model\Interfaces\Messages\EmailInterface $emailObject
     * @return boolean 
     */
    public function send(EmailInterface $emailObject) {
//        return false;
//        \Zend\Debug\Debug::dump($emailObject); exit;
        
        $emailMessage = new Message(); // Zend\Mail\Message
        
        $emailMessage->setEncoding('UTF-8');
        $emailMessage->setSubject($emailObject->getEmailSubject());
        $emailMessage->setFrom($emailObject->getEmailFromEmail(), $emailObject->getEmailFromName());
        $emailMessage->setTo($emailObject->getEmailToEmail(), $emailObject->getEmailToName());
        
        $emailMessage->setBody($this->getEmailBody($emailObject->getEmailBodyHtml()));
        
        $transport = new SmtpTransport(); // Zend\Mail\Transport\Smtp as SmtpTransport
        $transport->setOptions($this->smtpOptions);
        
        $emailObject->setEmailSentStatus(Email::STATUS_PENDING);
        $savedEmail = $this->mapper->addRow($emailObject);
        
        if($savedEmail['status']) {
            $emailObject = $savedEmail['object'];
            try {
                /*
                 * ZEND SMTP TRANSPORT doesn't return a result on success
                 * All we know is an exception is thrown on failed.
                 * This means the email status in the database will be set to 'failed' if an exception is raised
                 */

                $transport->send($emailMessage);
                $emailObject->setEmailSentStatus(Email::STATUS_SENT);
            } catch (\Exception $e) {
                $emailObject->setEmailSentStatus(Email::STATUS_FAILED);
                $emailObject->setEmailFailedReason($e->getMessage());
                // Log error $e
            } 
            
            $this->mapper->updateRow($emailObject, [
                    'updated' => Utils::timestamp(),
                    'email_sent_status' => $emailObject->getEmailSentStatus(),
                    'email_failed_reason' => $emailObject->getEmailFailedReason()
                ], [
                Configs::CONFIG_SEARCH_CRITERIA_POSITION_EQUAL => [
                    'column' => $emailObject->tablePrimaryKey(),
                    'value' => $emailObject->getEId()
                ]
            ]);
            
            return $emailObject->getEmailSentStatus() == Email::STATUS_SENT;
        }
        return false;
    }