The dbutil backup function is not working as what i expected

This is my code:

PHP Code:

$prefs = array(
            'tables'        => array(),  
            'ignore'        => array(),         
            'format'        => 'zip',              // gzip, zip, txt
            'filename'      => $filename.'.sql',       
            'add_drop'      => TRUE,      
            'add_insert'    => TRUE,     
            'newline'       => "\n"       
        );

        $backup = $this->dbutil->backup($prefs);        
        $backup_path = 'backups/db/';        
        $backup_file = FCPATH. $backup_path . $filename. "." . $prefs["format"]; 
        $backup_created = write_file( $backup_file, $backup); 


I am writing a backup data to a file , the string are not getting what i expected ;


Example output:

PHP Code:

INSERT INTO `104_activity_history` (`id`, `user_id`, `ip`, `done_by`, `date`, `activity`, `notification_status`, `data`) VALUES 
(9, 16, 127.0.0.1, 16, 2018-07-22 15:21:22, login, 0, ); 


This is what i am expecting:

INSERT INTO `104_activity_history` (`id`, `user_id`, `ip`, `done_by`, `date`, `activity`, `notification_status`, `data`) VALUES (9, 16, '127.0.0.1', 16, '2018-07-22 15:21:22', 'login', 0, '');

Look the value, The invertercomma is missing the output.