From what I understand, puppetd runs as root. As root, I launch

 puppetd --onetime --no-daemonize --verbose

So I don't understand why this doesn't work:

exec { "useradd -m testuser":
       path => "/bin:/usr/bin",
     }

I just get:

...Exec[useradd -m testuser]/returns: change from notrun to 0 failed:useradd -m testuser returned 1 instead of one of [0] at...

If I execute the command directly, it works just fine.

Any ideas?