ssh-add fails to add ed25519 keys

[Eero]

$ ssh-add
Identity added: /home/user/.ssh/id_rsa (/home/user/.ssh/id_rsa)
Error reading response length from authentication socket.
Could not add identity: /home/user/.ssh/id_ed25519

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: openssh-client 1:6.6p1-2ubuntu2
ProcVersionSignature: Ubuntu 3.13.0-61.100-generic 3.13.11-ckt22
Uname: Linux 3.13.0-61-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia
ApportVersion: 2.14.1-0ubuntu3.11
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Aug 7 14:11:29 2015
InstallationDate: Installed on 2014-05-16 (448 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
RelatedPackageVersions:
 ssh-askpass N/A
 libpam-ssh N/A
 keychain N/A
 ssh-askpass-gnome 1:6.6p1-2ubuntu2
SSHClientVersion: OpenSSH_6.6.1p1 Ubuntu-2ubuntu2, OpenSSL 1.0.1f 6 Jan 2014
SourcePackage: openssh
UpgradeStatus: No upgrade log present (probably fresh install)
modified.conffile..etc.ssh.moduli: [modified]
modified.conffile..etc.ssh.ssh.config: [modified]
mtime.conffile..etc.ssh.moduli: 2015-01-07T04:30:12.433235
mtime.conffile..etc.ssh.ssh.config: 2015-02-17T14:04:27.623708

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.