PQM --run always dies with lock file "pqm.lock" OSError

I am trying to set up PQM for the first time. I have Postfix+Procmail set up to pipe mail to the PQM user through ‘pqm --read’. This is working to place ‘patch.NNNNN’ files in the queue directory. However, whenever I try to run ‘pqm --run’, I always get this error at the end after it appears to have read the request file: OSError: [Errno 2] No such file or directory: '/home/bzrservertest/Queue/pqm/pqm.lock'

Here's the full output I get:

-----------------------------------------------------------------------------
bzrservertest@italia:~$ pqm --run -v -d
pqm [-1217083712] INFO: found groups {'MyProject_software_team': []}
pqm [-1217083712] INFO: group MyProject_software_team has members ['<email address hidden>', '<email address hidden>']
pqm [-1217083712] INFO: managing branch(s): file:///bzr/MyProject/
pqm [-1217083712] INFO: creating lockfile
pqm [-1217083712] INFO: trying script /home/bzrservertest/Queue/patch.1260236269
pqm [-1217083712] INFO: recieved email from cdb@italia (Colin D Bennett)
pqm [-1217083712] INFO: sender: cdb@italia (Colin D Bennett)
pqm [-1217083712] INFO: subject: Test 3
pqm [-1217083712] INFO: parsing commands
pqm [-1217083712] INFO: cleaning working directory
Traceback (most recent call last):
  File "/home/bzrservertest/Programs/pqm/bin/pqm", line 245, in <module>
    manager.run(script_queue)
  File "/home/bzrservertest/Programs/pqm/pqm/core.py", line 109, in run
    lockfile.release()
  File "/home/bzrservertest/Programs/pqm/pqm/lockfile.py", line 81, in release
    os.unlink(self.filename)
OSError: [Errno 2] No such file or directory: '/home/bzrservertest/Queue/pqm/pqm.lock'
-----------------------------------------------------------------------------

Let me also validate my understanding of incoming requests through mail. It seems to be a 2-step process:
(1) pqm --read is called by procmail, and it just stores to request directly in the queue directory, doing no real work.
(2) pqm --run must be called (via cron, or perhaps by the procmail trigger after pqm --read? It seems silly to keep polling with cron, which introduces unneeded delays) which actually processes the request.
Is this a correct understanding of how PQM works?