lftp Fatal error: Host key verification failed

When i connect to a remote SFTP server using lftp, i get following error

root@ok:~# lftp sftp://okuser@ws.nii.net:22
Password:
lftp okuser@ws.nii.net:~> set ftp:ssl-allow false
lftp okuser@ws.nii.net:~> mirror --verbose --use-pget-n=8 -c /home/okuser/www.site.com/docs /root/site-backup/
mirror: Fatal error: Host key verification failed.   
mkdir: Access failed: /root/site-backup/docs: File exists
lftp: MirrorJob.cc:242: void MirrorJob::JobFinished(Job*): Assertion `transfer_count>0' failed.
Aborted
root@ok:~#

The error is “Fatal error: Host key verification failed.”

To fix this, just SSH into this server with command

ssh -p 22 okuser@ws.nii.net

See lftp