deployment error: no passwd entry for app!

Hi, I'm having a problem when running `cap deploy`.  This is what I
get when it errors out.

 ** transaction: commit
  * executing `deploy:restart'
  * executing "sudo -p 'sudo password: ' -u app /project/current/
script/process/reaper"
    servers: ["web-01"]
    [web-01] executing command
*** [err :: web-01] sudo: no passwd entry for app!
    command finished
command "sudo -p 'sudo password: ' -u app /project/current/script/
process/reaper" failed on web-01


It looks like it's trying to run a sudo command as user app, for which
no such user exists.