diff --git a/test/functional/feature_dersig.py b/test/functional/feature_dersig.py
index 5dd6cb6cb2..fd46385cd4 100755
--- a/test/functional/feature_dersig.py
+++ b/test/functional/feature_dersig.py
@@ -104,7 +104,7 @@ class BIP66Test(BitcoinTestFramework):
         block.rehash()
         block.solve()
 
-        with self.nodes[0].assert_debug_log(expected_msgs=['{}, bad-version(0x00000002)'.format(block.hash)]):
+        with self.nodes[0].assert_debug_log(expected_msgs=[f'{block.hash}, bad-version(0x00000002)']):
             peer.send_and_ping(msg_block(block))
             assert_equal(int(self.nodes[0].getbestblockhash(), 16), tip)
             peer.sync_with_ping()
@@ -134,7 +134,7 @@ class BIP66Test(BitcoinTestFramework):
         block.rehash()
         block.solve()
 
-        with self.nodes[0].assert_debug_log(expected_msgs=['CheckInputScripts on {} failed with non-mandatory-script-verify-flag (Non-canonical DER signature)'.format(block.vtx[-1].hash)]):
+        with self.nodes[0].assert_debug_log(expected_msgs=[f'CheckInputScripts on {block.vtx[-1].hash} failed with non-mandatory-script-verify-flag (Non-canonical DER signature)']):
             peer.send_and_ping(msg_block(block))
             assert_equal(int(self.nodes[0].getbestblockhash(), 16), tip)
             peer.sync_with_ping()