diff --git a/dataset/youtubevis.py b/dataset/youtubevis.py
index babbf0e..8abe40a 100644
--- a/dataset/youtubevis.py
+++ b/dataset/youtubevis.py
@@ -38,7 +38,7 @@ def __init__(self, videodir, annfile, size, seq_length, seq_sampler, transform=N
         
         self.index = []
         for video_id in self.videos.keys():
-            for frame in range(len(self.videos[video_id])):
+            for frame in range(len(self.videos[video_id]['file_names'])):
                 self.index.append((video_id, frame))
                 
     def __len__(self):
@@ -120,4 +120,4 @@ def __call__(self, imgs, segs):
             imgs = F.hflip(imgs)
             segs = F.hflip(segs)
         
-        return imgs, segs
\ No newline at end of file
+        return imgs, segs