title: What does "Failed to execute operation: Invalid argument" mean when running systemctl enable?
body: <p>I've created a systemd service file (specifically for svnserve; I'm actually using the example from here <a href="https://stackoverflow.com/a/40584047/464087">https://stackoverflow.com/a/40584047/464087</a>), and when I enable it, typing</p>
  
  <pre><code>sudo systemctl enable svnserve
  </code></pre>
  
  <p>I get the response </p>
  
  <pre><code>Failed to execute operation: Invalid argument
  </code></pre>
  
  <p>Running </p>
  
  <pre><code>sudo systemctl status svnserve
  </code></pre>
  
  <p>yields</p>
  
  <pre><code>● svnserve.service - Subversion protocol daemon
     Loaded: loaded (/etc/systemd/system/svnserve.service; enabled; vendor preset: enabled)
     Active: inactive (dead)
  </code></pre>
  
  <p>not giving me any clue about anything being wrong. I can then start the service without any error, and it seems to be running as expected, and after starting systemctl status I still get no clue about anything being wrong:</p>
  
  <pre><code>● svnserve.service - Subversion protocol daemon
     Loaded: loaded (/etc/systemd/system/svnserve.service; enabled; vendor preset: enabled)
     Active: active (running) since Tue 2018-01-09 22:10:14 UTC; 6s ago
    Process: 9677 ExecStart=/usr/bin/svnserve $DAEMON_ARGS (code=exited, status=0/SUCCESS)
   Main PID: 9678 (svnserve)
      Tasks: 1
     Memory: 964.0K
        CPU: 2ms
     CGroup: /system.slice/svnserve.service
             └─9678 /usr/bin/svnserve --daemon --pid-file /run/svnserve/svnserve.pid --root /srv/svn/repos --log-file /var/log/svnserve/svnserve.log
  </code></pre>
  
  <p>So what does this error message mean? And to which level of things is "invalid argument" supposed to apply? An argument to the svnserve command? Some property in the service file? A command line argument to the servicectl command itself?</p>
  
  <p>FWIW this is on a Ubuntu 16.04 LTS server.</p>