The following code throws a SystemError exception. cell_get_contents in
Objects\cellobject.c should check for a null op->ob_ref value and throw
an appropriate exception.

>>> def oops():
	def f(): cell
	f.func_closure[0].cell_contents
	cell = None

	
>>> oops()

Traceback (most recent call last):
  File "<pyshell#9>", line 1, in <module>
    oops()
  File "<pyshell#8>", line 3, in oops
    f.func_closure[0].cell_contents
SystemError: error return without exception set
>>>