builtins.KeyError: 'stat' when file is not existing

[tardyp]

As per:
https://build.contextgarden.net/#/builders/13/builds/55

builtins.KeyError: 'stat'
/data1/buildbot/install-2.x/sandbox/lib/python3.7/site-packages/buildbot/process/buildstep.py:567 in startStep
566                    self._running = True
567                    self.results = yield self.run()
568                finally:
/data1/buildbot/install-2.x/sandbox/lib/python3.7/site-packages/twisted/internet/defer.py:1418 in _inlineCallbacks
1417            else:
1418                result = g.send(result)
1419        except StopIteration as e:
/data1/buildbot/install-2.x/sandbox/lib/python3.7/site-packages/buildbot/process/buildstep.py:697 in run
696                self._start_deferred.callback(results)
697            results = yield self._start_deferred
698        finally:
/data1/buildbot/install-2.x/sandbox/lib/python3.7/site-packages/twisted/internet/defer.py:1416 in _inlineCallbacks
1415            if isFailure:
1416                result = result.throwExceptionIntoGenerator(g)
1417            else:
/data1/buildbot/install-2.x/sandbox/lib/python3.7/site-packages/twisted/python/failure.py:512 in throwExceptionIntoGenerator
511        # is done in _findFailure.
512        return g.throw(self.type, self.value, self.tb)
513
/data1/buildbot/install-2.x/sandbox/lib/python3.7/site-packages/buildbot/steps/transfer.py:397 in uploadSources
396                for source in sources:
397                    result = yield self.startUpload(source, masterdest)
398                    if result == FAILURE:
/data1/buildbot/install-2.x/sandbox/lib/python3.7/site-packages/twisted/internet/defer.py:654 in _runCallbacks
653                    try:
654                        current.result = callback(current.result, *args, **kw)
655                        if current.result is current:
/data1/buildbot/install-2.x/sandbox/lib/python3.7/site-packages/buildbot/steps/transfer.py:342 in checkStat
341        def checkStat(_):
342            s = cmd.updates['stat'][-1]
343            if stat.S_ISDIR(s[stat.ST_MODE]):
builtins.KeyError: 'stat'

in the worker implementation:

       try:
            stat = os.stat(filename)
            self.sendStatus({'stat': tuple(stat)})
            self.sendStatus({'rc': 0})
        except OSError as e:
            log.msg("StatFile {0} failed: {1}".format(filename, e))
            self.sendStatus(
                {'header': '{0}: {1}: {2}'.format(self.header, e.strerror, filename)})
            self.sendStatus({'rc': e.errno})

when os.stat fail, no 'stat' status is send, but the master will anyway check for 'stat' first