diff --git a/library/train_util.py b/library/train_util.py
index 1d616376..534771ad 100644
--- a/library/train_util.py
+++ b/library/train_util.py
@@ -1009,6 +1009,10 @@ class BaseDataset(torch.utils.data.Dataset):
             return None, None, None
 
         npz = np.load(npz_file)
+        if "latents" not in npz:
+            print(f"error: npz is old format. please re-generate {npz_file}")
+            return None, None, None
+
         latents = npz["latents"]
         original_size = npz["original_size"].tolist()
         crop_left_top = npz["crop_left_top"].tolist()