diff --git a/electrum/blockchain.py b/electrum/blockchain.py
index a3953a112..082fee5ee 100644
--- a/electrum/blockchain.py
+++ b/electrum/blockchain.py
@@ -45,7 +45,7 @@ def serialize_header(header_dict: dict) -> str:
     s = int_to_hex(header_dict['version'], 4) \
         + rev_hex(header_dict['prev_block_hash']) \
         + rev_hex(header_dict['merkle_root']) \
-        + rev_hex(res.get('claim_trie_root')) \
+        + rev_hex(header_dict['claim_trie_root']) \
         + int_to_hex(int(header_dict['timestamp']), 4) \
         + int_to_hex(int(header_dict['bits']), 4) \
         + int_to_hex(int(header_dict['nonce']), 4)