Pigeonhole Sieve Config Warning

[dovecot]

In the sieve plugin settings i have
  plugin {
    ...
    sieve_user_email = %u
  }
Variables pages says
  %u = full username (e.g. user@domain)

This config seems to work in practice, the email is from: the users 
email address.

However on starting dovecot im getting a warning twice:
  dovecot[35893]: managesieve: Warning: sieve: Invalid address value for 
setting `sieve_user_email': Missing domain
  dovecot[35893]: master: Dovecot v2.3.8 (9df20d2db) starting up for 
imap, lmtp, sieve
  dovecot[35897]: managesieve: Warning: sieve: Invalid address value for 
setting `sieve_user_email': Missing domain

So is there really an issue? Or just an improper warning?