diff --git a/scripts/tools/perplexity.py b/scripts/tools/perplexity.py
index a410320..84b2640 100644
--- a/scripts/tools/perplexity.py
+++ b/scripts/tools/perplexity.py
@@ -44,8 +44,8 @@ def process_file(
 
         for seq in batch_encoded:
             pad_len = max_len - len(seq)
-            padded_seq = [tokenizer.pad_id] * pad_len + seq
-            mask = [False] * pad_len + [True] * len(seq)
+            padded_seq = seq + [tokenizer.pad_id] * pad_len
+            mask = [True] * len(seq) + [False] * pad_len
             padded_ids.append(padded_seq)
             masks.append(mask)