title: TypeError: 'NoneType' object is not iterable occurring only on big data
body: <p>I get the error: TypeError: 'NoneType' object is not iterable 
  when i try to geocode over 200K IPs  using the following code excerpt. Please note that when I geocode just 100K IPs no errors are occurring:</p>
  
  <pre><code>#writing a function that takes index, geocodes the ip individually for multithreading 
  def new_geocode_ip_ver2(ipandresponse):
          if ipandresponse[1]!='0':
              match=geolite2.lookup(ipandresponse[0])
              if match is not None: 
                  lat, lng = match.location 
              else:
                  lat="GC failed"
                  lng="GC failed"
          else:
              lat="Non-pingable, no GC"
              lng="Non-pingable, no GC"
  
          return lat, lng
  
  #spawn threads for geocoding IPs
  pool=ThreadPool(200)
  latlng_result=pool.map(new_geocode_ip_ver2,ip_response)
  pool.close()
  pool.join()
  </code></pre>
  
  <p>The error is message is provided below. Can anyone tell me why this error is occuring on over 100k IP only. I am trying to build my program in such a way that I will be able to geocode a million + IPs. Do I have some kind of memory leak by any chance?</p>
  
  <pre><code>---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-6-26e36f25645e&gt; in &lt;module&gt;()
       32 #spawn threads for geocoding IPs
       33 pool=ThreadPool(200)
  ---&gt; 34 latlng_result=pool.map(new_geocode_ip_ver2,ip_response)
       35 pool.close()
       36 pool.join()
  
  C:\Users\sibag\Anaconda2\lib\multiprocessing\pool.pyc in map(self, func, iterable, chunksize)
      249         '''
      250         assert self._state == RUN
  --&gt; 251         return self.map_async(func, iterable, chunksize).get()
      252 
      253     def imap(self, func, iterable, chunksize=1):
  
  C:\Users\sibag\Anaconda2\lib\multiprocessing\pool.pyc in get(self, timeout)
      565             return self._value
      566         else:
  --&gt; 567             raise self._value
      568 
      569     def _set(self, i, obj):
  
  TypeError: 'NoneType' object is not iterable
  </code></pre>