Errors: example when doing apt-clean in the repo area of the web interface

Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fa1d55258c8>
Traceback (most recent call last):
  File "/usr/lib/python3.5/weakref.py", line 117, in remove
TypeError: 'NoneType' object is not callable
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fa1d55258c8>
Traceback (most recent call last):
  File "/usr/lib/python3.5/weakref.py", line 117, in remove
TypeError: 'NoneType' object is not callable
Reading package lists...
Done...