last indexerror: tuple index out of range

Here is a reproducible example:

foo = zip([1,2,3],[4,5,6])
last(foo)
last(foo)

The output in my repl looks like this

In[43]: foo = zip([1,2,3],[4,5,6])
In[44]: last(foo)
Out[44]: (3, 6)
In[45]: last(foo)
Traceback (most recent call last):
  File "/Users/mlindon/anaconda3/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 2963, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-45-06bc2d3f7f9e>", line 1, in <module>
    last(foo)
  File "/Users/mlindon/anaconda3/lib/python3.6/site-packages/toolz/itertoolz.py", line 398, in last
    return tail(1, seq)[0]
IndexError: tuple index out of range

Start with toolz/itertoolz.py at the last function and inspect how it delegates to tail(1, seq). Reproduce the example with an exhausted zip iterator, then check the surrounding tests or add a focused regression test to establish the expected behavior for repeated calls. Done means the exhausted-iterator case has a defined, tested result.