ED25519/ED448: unsupported algorithm

I've installed bind9utils 9.11.4+dfsg-4~bpo9+1 from stretch-backports which includes support for the ED25519 and ED448 signing algorithm, yet dnssec-keygen apparently can't handle it:

$ dnssec-keygen -a ED25519 test
dnssec-keygen: fatal: unsupported algorithm: 15
$ dnssec-keygen -a ED448 test
dnssec-keygen: fatal: unsupported algorithm: 16

$ dnssec-keygen -h 2>&1 | grep Version
Version: 9.11.4-4~bpo9+1-Debian