Python error? on files that seem to be unable to compress

[fostimus]

I get this on some files after compressing, and I also get this on some files that just won't compress. Can anyone tell me what this error means?

Exception ignored in: <function Worker.__del__ at 0x113199790>
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/trimage/trimage.py", line 414, in __del__
  File "/usr/local/Cellar/trimage/1.0.6_2/lib/python3.9/site-packages/trimage/ThreadPool/ThreadPool.py", line 172, in shutdown
TypeError: 'NoneType' object is not callable
Substitution replacement not terminated at (eval 2) line 1.