Inspec postgres_session question

I’m trying to use postgres_session, like so:

  its('output') { should eq('') }

But I get the following when running from inside TK:

```  Command service
     ✔  postgresql status stdout should match /9.3\/main \(port 5432\): online/
     ✔  postgresql status stderr should eq ""
     ✔  postgresql status exit_status should eq 0
  Can't read
     ∅  undefined method `output' for "output":String```

Any ideas what's going wrong?