No blocks message

Here is an example org file:
```org
#+property: header-args :tangle test.nix

* Some title goes here
  #+begin_src nix
    some nix code goes here
  #+end_src
```
When running `:OrgTangle` on the above file, I get the `No blocks` message below the status line. Is this a bug, or am I missing something?