Invalid address: (addAnAddress to):

Fatal error: Uncaught exception 'phpmailerException' with message 'Invalid address: (addAnAddress to): ' in /mnt/data/accounts/d/davidvotava/data/www/maestro/PHPMailer/class.phpmailer.php:881 Stack trace: #0 /mnt/data/accounts/d/davidvotava/data/www/maestro/PHPMailer/class.phpmailer.php(821): PHPMailer->addOrEnqueueAnAddress('to', false, '') #1 /mnt/data/accounts/d/davidvotava/data/www/maestro/application/models/mailer.php(179): PHPMailer->addAddress(false) #2 /mnt/data/accounts/d/davidvotava/data/www/maestro/application/models/mailer.php(18): Mailer->send(false, 'T?? Maestro: Re...', '<p>Dobr?? den,<...') #3 /mnt/data/accounts/d/davidvotava/data/www/maestro/application/controllers/registrace.php(30): Mailer->registration_email(false) #4 [internal function]: Registrace->index() #5 /mnt/data/accounts/d/davidvotava/data/www/maestro/system/core/CodeIgniter.php(359): call_user_func_array(Array, Array) #6 /mnt/data/accounts/d/davidvotava/data/www/maestro/index.php(202): require_once('/mnt/data/accou...') #7 {main} thrown in /mnt/data/accounts/d/davidvotava/data/www/maestro/PHPMailer/class.phpmailer.php on line 881