If you add the "if __name__ == '__main__':" guard after defining the target function, as specified in the multiprocessing doc, you will get a traceback much as you expect:

Traceback (most recent call last):
  File "F:\Python\mypy\tem.py", line 12, in <module>
    for new_val in pool.imap(add_one, value_iter):
  File "C:\Programs\Python35\lib\multiprocessing\pool.py", line 695, in next
    raise value
  File "C:\Programs\Python35\lib\multiprocessing\pool.py", line 380, in _handle_tasks
    for i, task in enumerate(taskseq):
  File "C:\Programs\Python35\lib\multiprocessing\pool.py", line 286, in <genexpr>
    self._taskqueue.put((((result._job, i, func, (x,), {})
  File "F:\Python\mypy\tem.py", line 10, in <genexpr>
    value_iter = (int(v) for v in values)
ValueError: invalid literal for int() with base 10: 'foo'

I have seem this bug of omission multiple times on Stackoverflow.