Exception for _last_command_byte

[Brettles]

Under some circumstances (it seems when there are many packets in flight from a remote peer) there is an exception raised:

Traceback (most recent call last):
  File "/home/ubuntu/python/construct/lib/containers.py", line 93, in __getattr__
    return self[name]
KeyError: '_last_command_byte'

I can see in the code where this is being used but I don't have enough knowledge to figure out how to solve this.