diff --git a/render.py b/render.py
index 1f88a6c..b551831 100644
--- a/render.py
+++ b/render.py
@@ -451,6 +451,12 @@ def render_all(options):
             for cmd in cmds:
                 #print(" ".join([str(x) for x in cmd]))
                 subprocess.call(cmd)
+            ft = ox.avinfo(fragment_prefix / "front-5.1.mp4")['duration']
+            bt = ox.avinfo(fragment_prefix / "back.mp4")['duration']
+            if ft != bt:
+                print(ft, fragment_prefix / "front-5.1.mp4")
+                print(bt, fragment_prefix / "back.mp4")
+                sys.exit(-1)
             shutil.move(fragment_prefix / "front-5.1.mp4", fragment_prefix / "front.mp4")
             for fn in (
                 "audio-5.1.mp4", "fl.wav", "fr.wav", "fc.wav", "lfe.wav", "bl.wav", "br.wav",
@@ -458,6 +464,7 @@ def render_all(options):
                 fn = fragment_prefix / fn
                 if os.path.exists(fn):
                     os.unlink(fn)
+
     print("Duration - Target: %s Actual: %s" % (target_position, position))
     with open(_cache, "w") as fd:
         json.dump(_CACHE, fd)