diff -urNp old/modules/optflow/samples/motempl.py new/modules/optflow/samples/motempl.py
--- old/modules/optflow/samples/motempl.py	2016-12-23 12:09:43.000000000 +0100
+++ new/modules/optflow/samples/motempl.py	2017-02-27 09:04:30.218887521 +0100
@@ -37,7 +37,8 @@ if __name__ == '__main__':
         sys.exit(1)
     ret, frame = cam.read()
     if ret == False:
-        break
+        print("could not read from " + str(video_src) + " !\n")
+        sys.exit(1)
     h, w = frame.shape[:2]
     prev_frame = frame.copy()
     motion_history = np.zeros((h, w), np.float32)