>>> import abc
>>> class A(object, metaclass=abc.ABCMeta):
...     pass
>>> issubclass([], A)
Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    issubclass([], A)
  File "c:\Python32\lib\abc.py", line 137, in __subclasscheck__
    if subclass in cls._abc_cache:
  File "c:\Python32\lib\_weakrefset.py", line 69, in __contains__
    return ref(item) in self.data
TypeError: cannot create weak reference to 'list' object

I should be able to check whether an object is a subclass of something without confirming it's a type first. I think this call should just return False.