--- a/thread.py	2016-11-01 02:09:20.000000000 +0100
+++ b/thread.py	2016-11-09 17:02:58.363326699 +0100
@@ -66,7 +66,12 @@
                 work_item.run()
                 # Delete references to object. See issue16284
                 del work_item
+                work_queue.task_done()
                 continue
+
+            # We can safely call task_done here, if we should shutdown, we will
+            # just put another None object back into the queue and exit.
+            work_queue.task_done()
             executor = executor_reference()
             # Exit if:
             #   - The interpreter is shutting down OR
@@ -124,7 +129,8 @@
         # TODO(bquinlan): Should avoid creating new threads if there are more
         # idle threads than items in the work queue.
         num_threads = len(self._threads)
-        if num_threads < self._max_workers:
+        if self._work_queue.unfinished_tasks > num_threads \
+                and num_threads < self._max_workers:
             thread_name = '%s_%d' % (self._thread_name_prefix or self,
                                      num_threads)
             t = threading.Thread(name=thread_name, target=_worker,