diff --git a/cvat/apps/engine/media_extractors.py b/cvat/apps/engine/media_extractors.py
index 6d94c0945c17..d9eead2b9f7e 100644
--- a/cvat/apps/engine/media_extractors.py
+++ b/cvat/apps/engine/media_extractors.py
@@ -70,7 +70,7 @@ def get_image_size(self, i):
         pass
 
     def __len__(self):
-        return (self._stop - self._start) // self._step
+        return len(self.frame_range)
 
     @property
     def frame_range(self):