diff --git a/tools/lib/framereader.py b/tools/lib/framereader.py
index 7c01992a28..74747f5eb9 100644
--- a/tools/lib/framereader.py
+++ b/tools/lib/framereader.py
@@ -490,7 +490,7 @@ class GOPFrameReader(BaseFrameReader):
 
       ret = decompress_video_data(rawdat, self.vid_fmt, self.w, self.h, pix_fmt)
       ret = ret[skip_frames:]
-      assert ret.shape[0] == num_frames
+      assert ret.shape[0] == num_frames, (ret.shape[0], num_frames)
 
       for i in range(ret.shape[0]):
         self.frame_cache[(frame_b+i, pix_fmt)] = ret[i]