owner error: builtins.TypeError: unhashable type: 'list'

Not sure if I'm doing something wrong or a bug. I have a scheduler that worked until I added a properties value with owners.

Defined like `DOC_SCHEDULER = schedulers.SingleBranchScheduler(`

```
properties = {
'owner': ['my@email.com']
}
```

But when I run `buildbot-worker restart` the job doesn't finish. In the UI it's not red or green but purple (have not seen that before) and the HTML shows this error:

```
builtins.TypeError: unhashable type: 'list'
/usr/lib/python3/dist-packages/buildbot/process/build.py:334 in startBuild
333 try:
334 self.setupBuild() # create .steps
335 except Exception:
/usr/lib/python3/dist-packages/buildbot/process/build.py:486 in setupBuild
485 # gather owners from build requests
486 owners.update({r.properties['owner'] for r in self.requests
487 if "owner" in r.properties})
/usr/lib/python3/dist-packages/buildbot/process/build.py:486 in
485 # gather owners from build requests
486 owners.update({r.properties['owner'] for r in self.requests
487 if "owner" in r.properties})
builtins.TypeError: unhashable type: 'list'
```

Is this a bug or did I misunderstand how to apply the information in this doc: https://docs.buildbot.net/latest/manual/configuration/schedulers.html?highlight=owners