dovecot and argon2 encryption

[Jerry]

I am using a FreeBSD 11-2 amd/64 system with dovecot version 2.3.4 installed.
I was playing around with different encryption schemes.

doveadm pw -l
SHA1 SSHA512 BLF-CRYPT PLAIN HMAC-MD5 OTP SHA512 SHA RPA DES-CRYPT CRYPT SSHA
 MD5-CRYPT SKEY PLAIN-MD4 PLAIN-MD5 SCRAM-SHA-1 LANMAN SHA512-CRYPT CLEAR
 CLEARTEXT SSHA256 NTLM MD5 PBKDF2 SHA256 CRAM-MD5 PLAIN-TRUNC SHA256-CRYPT
 SMD5 DIGEST-MD5 LDAP-MD5

There is no mention of "argon2" shown. Now, from the command line I can enter
this command:

~ $ echo -n "Secret-Password" | argon2 somesalt
Type:           Argon2i
Iterations:     3
Memory:         4096 KiB
Parallelism:    1
Hash:
e6432f595e999988c7c54c30d530b0fc7d9953510e5ccf295359258f4ea22a3d
Encoded:
$argon2i$v=19$m=4096,t=3,p=1$c29tZXNhbHQ$5kMvWV6ZmYjHxUww1TCw/H2ZU1EOXM8pU1klj06iKj0
0.039 seconds Verification ok

However, this fails:

doveadm pw -p "Secret-Password" -s argon2
Fatal: Unknown scheme: ARGON2

I have tried different ways to enter "argon2", but nothing works. Can
someone tell me what I am doing wrong?

Thanks :)

-- 
Jerry