scheduler.py: "Task exception was never retrieved"

[CRASNY]

I get the following error at times after a scheduled event runs:

[29/03/2017 01:00] INFO scheduler run_coro 240: Running 'dump' in 261733281341374469.
[29/03/2017 01:00] INFO scheduler run_coro 240: Running 'announce dump' in 261733281341374469.
Task exception was never retrieved
future: <Task finished coro=<Scheduler.queue_manager() done, defined at C:\Users\Craig\Documents\Red-DiscordBot\cogs\scheduler.py:244> exception=KeyError(1490763641,)>
Traceback (most recent call last):
  File "C:\Program Files\Python35\lib\asyncio\tasks.py", line 239, in _step
    result = coro.send(None)
  File "C:\Users\Craig\Documents\Red-DiscordBot\cogs\scheduler.py", line 278, in queue_manager
    del self.to_kill[start_time]
KeyError: 1490763641

Does anyone know what causes it and how I can fix it?