diff --git a/lib/blockchain.py b/lib/blockchain.py
index 9f26f648..a913ef23 100644
--- a/lib/blockchain.py
+++ b/lib/blockchain.py
@@ -215,12 +215,13 @@ class Blockchain(util.PrintError):
 
     def save_chunk(self, index, chunk):
         filename = self.path()
+        height = index * NetworkConstants.CHUNK_SIZE - self.checkpoint
         with open(filename, 'rb') as f:
-            d = self._height_to_offset(f, index * NetworkConstants.CHUNK_SIZE - self.checkpoint)
+            d = self._height_to_offset(f, height)
         if d < 0:
             chunk = chunk[-d:]
             d = 0
-        self.write(chunk, d, index > len(self.checkpoints))
+        self.write(chunk, height, index > len(self.checkpoints))
         self.swap_with_parent()
 
     def swap_with_parent(self):