diff --git a/cvpubsubs/__init__.py b/cvpubsubs/__init__.py
index abeeedb..f0ede3d 100644
--- a/cvpubsubs/__init__.py
+++ b/cvpubsubs/__init__.py
@@ -1 +1 @@
-__version__ = '0.4.0'
+__version__ = '0.4.1'
diff --git a/cvpubsubs/webcam_pub/frame_handler.py b/cvpubsubs/webcam_pub/frame_handler.py
index 87a61c6..2eb1372 100644
--- a/cvpubsubs/webcam_pub/frame_handler.py
+++ b/cvpubsubs/webcam_pub/frame_handler.py
@@ -73,7 +73,9 @@ class VideoHandlerThread(threading.Thread):
             if frame is not None:
                 frame = frame
                 for c in self.callbacks:
-                    frame = c(frame, self.cam_id)
+                    frame_c = c(frame, self.cam_id)
+                    if frame_c is not None:
+                        frame = frame_c
             msg_owner = sub_owner.get()
         sub_owner.release()
         sub_cam.release()
diff --git a/cvpubsubs/window_sub/cv_window_sub.py b/cvpubsubs/window_sub/cv_window_sub.py
index 5bd9c03..da1533e 100644
--- a/cvpubsubs/window_sub/cv_window_sub.py
+++ b/cvpubsubs/window_sub/cv_window_sub.py
@@ -69,25 +69,26 @@ class SubscriberWindows(object):
 
     def _display_frames(self, frames, win_num):
         for f in range(len(frames)):
-            if frames[f].dtype.num == 17 or len(frames[f].shape) > 3:  # detect nested
-                self._display_frames(frames[f], win_num)
+            # detect nested:
+            if isinstance(frames[f], (list, tuple)) or frames[f].dtype.num == 17 or len(frames[f].shape) > 3:
+                win_num = self._display_frames(frames[f], win_num)
             else:
                 cv2.imshow(self.window_names[win_num % len(self.window_names)] + " (press ESC to quit)", frames[f])
                 win_num += 1
+        return win_num
 
     def update_window_frames(self):
         win_num = 0
         for i in range(len(self.input_vid_global_names)):
-            if self.input_vid_global_names[i] in self.frame_dict and not isinstance(self.frame_dict[
-                                                                                        self.input_vid_global_names[i]],
-                                                                                    NoData):
+            if self.input_vid_global_names[i] in self.frame_dict and \
+                    not isinstance(self.frame_dict[self.input_vid_global_names[i]], NoData):
                 if len(self.callbacks) > 0 and self.callbacks[i % len(self.callbacks)] is not None:
                     frames = self.callbacks[i % len(self.callbacks)](self.frame_dict[self.input_vid_global_names[i]])
                 else:
                     frames = self.frame_dict[self.input_vid_global_names[i]]
                 if isinstance(frames, np.ndarray) and len(frames.shape) <= 3:
                     frames = [frames]
-                self._display_frames(frames, win_num)
+                win_num = self._display_frames(frames, win_num)
 
     # todo: figure out how to get the red x button to work. Try: https://stackoverflow.com/a/37881722/782170
     def loop(self):
diff --git a/tests/test_sub_win.py b/tests/test_sub_win.py
index 594740c..7adf8a4 100644
--- a/tests/test_sub_win.py
+++ b/tests/test_sub_win.py
@@ -18,7 +18,7 @@ def print_keys_thread():
     msg_cmd = ''
     while msg_cmd != 'quit':
         key_chr = sub_key.get(sub_key)  # type: np.ndarray
-        WinCtrl.key_pub.publish(None) # consume data
+        WinCtrl.key_pub.publish(None)  # consume data
         if key_chr is not None:
             print("key pressed: " + str(key_chr))
         msg_cmd = sub_cmd.get()
@@ -92,7 +92,14 @@ class TestSubWin(ut.TestCase):
 
     def test_nested_frames(self):
         def nest_frame(frame, cam_id):
-            frame = np.asarray([[[[[[frame]]]]]])
+            frame = np.asarray([[[[[[frame]]]]], [[[[[frame]]], [[[frame]]]]]])
             return frame
 
-        w.VideoHandlerThread(callbacks=[nest_frame] + w.display_callbacks).display()
\ No newline at end of file
+        v = w.VideoHandlerThread(callbacks=[nest_frame] + w.display_callbacks)
+        v.start()
+
+        SubscriberWindows(window_names=[str(i) for i in range(3)],
+                          video_sources=[str(0)]
+                          ).loop()
+
+        v.join()