diff --git a/telethon/update_state.py b/telethon/update_state.py
index 1ac2e00d..98345cdf 100644
--- a/telethon/update_state.py
+++ b/telethon/update_state.py
@@ -39,19 +39,15 @@ class UpdateState:
         return not self._updates.empty()
 
     def poll(self, timeout=None):
-        """Polls an update or blocks until an update object is available.
-           If 'timeout is not None', it should be a floating point value,
-           and the method will 'return None' if waiting times out.
+        """
+        Polls an update or blocks until an update object is available.
+        If 'timeout is not None', it should be a floating point value,
+        and the method will 'return None' if waiting times out.
         """
         try:
-            update = self._updates.get(timeout=timeout)
+            return self._updates.get(timeout=timeout)
         except Empty:
-            return
-
-        if isinstance(update, Exception):
-            raise update  # Some error was set through (surely StopIteration)
-
-        return update
+            return None
 
     def get_workers(self):
         return self._workers
@@ -60,27 +56,24 @@ class UpdateState:
         """Changes the number of workers running.
            If 'n is None', clears all pending updates from memory.
         """
-        self.stop_workers()
-        self._workers = n
-        if n is not None:
+        if n is None:
+            self.stop_workers()
+        else:
+            self._workers = n
             self.setup_workers()
 
     workers = property(fget=get_workers, fset=set_workers)
 
     def stop_workers(self):
         """
-        Raises "StopIterationException" on the worker threads to stop
-        them, and also clears all the workers/updates from the lists.
+        Waits for all the worker threads to stop.
         """
-        if self._workers:
-            with self._updates_lock:
-                # Insert at the beginning so the very next poll causes an error
-                # on all the worker threads
-                # TODO Should this reset the pts and such?
-                while self._updates:
-                    self._updates.get()
-                for _ in range(self._workers):
-                    self._updates.put(StopIteration())
+        # Put dummy ``None`` objects so that they don't need to timeout.
+        n = self._workers
+        self._workers = None
+        with self._updates_lock:
+            for _ in range(n):
+                self._updates.put(None)
 
         for t in self._worker_threads:
             t.join()
@@ -103,7 +96,7 @@ class UpdateState:
             thread.start()
 
     def _worker_loop(self, wid):
-        while True:
+        while self._workers is not None:
             try:
                 update = self.poll(timeout=UpdateState.WORKER_POLL_TIMEOUT)
                 if update and self.handler: