Buildbot step hangs if log contains special characters

One of my build steps, while following log file, produces some unprintable character. In such case the step seems to hang on closing the connection (after the command was executed). When step timeout happens (or after manually hitting cancel):

```Traceback (most recent call last):
File "/home/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 501, in errback
self._startRunCallbacks(fail)
File "/home/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 568, in _startRunCallbacks
self._runCallbacks()
File "/home/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 654, in _runCallbacks
current.result = callback(current.result, *args, **kw)
File "/home/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1475, in gotResult
_inlineCallbacks(r, g, status)
--- ---
File "/home/buildbot/sandbox/local/lib/python2.7/site-packages/buildbot/process/buildstep.py", line 577, in startStep
self.results = yield self.run()
File "/home/buildbot/sandbox/local/lib/python2.7/site-packages/buildbot/process/buildstep.py", line 707, in run
results = yield self._start_deferred
File "/home/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
result = g.send(result)
File "/home/buildbot/sandbox/local/lib/python2.7/site-packages/buildbot/process/buildstep.py", line 707, in run
results = yield self._start_deferred
exceptions.TypeError: 'unicode' does not have the buffer interface
```

Example is one of last steps here ("Power off: odroidxu3_1"):
https://krzk.eu/#/builders/1/builds/2573

The command which causes it is a Python code running pexpect which spawns the picocom (serial/terminal emulation program). The special character goes to picocom which stores it under log file. The log file is read by Buildbot worker. The pexpect and picocom work fine, they finish work and step should finish itself as well... but it does not. It waits and waits and finally prints error as above.

Everything works fine if the log is not created.

Configuration:
Master: Python 2.7.6, Buildbot 1.3.0 (from pip under sandbox, Ubuntu, x86)
Worker: Python 3.7.0, Buildbot 1.3.0 (from pip under sandbox, Arch ARM Linux, ARMv7)

The step which causes it is here:
https://github.com/krzk/tools/blob/1a299defb4d8bbef8a63ddeaef7d578585b28684/buildbot/master_boot.py#L382

Best regards,
Krzysztof