diff --git a/blockchain.py b/blockchain.py
index 6f8e38f..ae9fb99 100644
--- a/blockchain.py
+++ b/blockchain.py
@@ -30,7 +30,7 @@ class Transaction:
             self.sender + \
             self.receiver + \
             self.amount.to_bytes(8, "big") + \
-            self.transaction_fee(8, "big")
+            self.transaction_fee.to_bytes(8, "big")
         try:
             sender_pubkey.verify(self.signature, msg)
         except InvalidSignature: