Why does lftp fail with the error "gnutls_handshake: A record packet with illegal version was received"?

Issue

• lftp fails when running ls command:

$ lftp -e "debug 13; set ftp:ssl-force true; set ftp:ssl-protect-data true; set ftp:ssl-protect-list true; set ftp:ssl-auth SSL; set ssl:verify-certificate no;" -p 990 -u myuser ftps://myserver
Password: 
lftp myuser@myserver:~> ls       
FileCopy(0x1ca10b80) enters state INITIAL
FileCopy(0x1ca10b80) enters state DO_COPY
---- dns cache hit
---- Connecting to myserver (1.2.3.4) port 990
GNUTLS: HSK[1cd040e0]: Keeping ciphersuite: RSA_AES_128_CBC_SHA1
GNUTLS: HSK[1cd040e0]: Keeping ciphersuite: RSA_3DES_EDE_CBC_SHA1
GNUTLS: HSK[1cd040e0]: Keeping ciphersuite: RSA_ARCFOUR_SHA1
GNUTLS: HSK[1cd040e0]: Keeping ciphersuite: RSA_ARCFOUR_MD5
GNUTLS: HSK[1cd040e0]: Keeping ciphersuite: DHE_DSS_AES_128_CBC_SHA1
GNUTLS: HSK[1cd040e0]: Keeping ciphersuite: DHE_DSS_3DES_EDE_CBC_SHA1
GNUTLS: HSK[1cd040e0]: Keeping ciphersuite: DHE_RSA_AES_128_CBC_SHA1
GNUTLS: HSK[1cd040e0]: Keeping ciphersuite: DHE_RSA_3DES_EDE_CBC_SHA1
GNUTLS: HSK[1cd040e0]: CLIENT HELLO was send [61 bytes]
GNUTLS: REC[1cd040e0]: Sending Packet[0] Handshake(22) with length: 61
GNUTLS: REC[1cd040e0]: Sent Packet[1] Handshake(22) with length: 66
**** gnutls_handshake: A record packet with illegal version was received.
---- Closing control socket
ls: Fatal error: gnutls_handshake: A record packet with illegal version was received.