diff --git a/electrum/paymentrequest.py b/electrum/paymentrequest.py
index 43fab125c..15c7b4b68 100644
--- a/electrum/paymentrequest.py
+++ b/electrum/paymentrequest.py
@@ -403,7 +403,7 @@ def verify_cert_chain(chain):
             hashBytes = bytearray(hashlib.sha512(data).digest())
             verify = pubkey.verify(sig, x509.PREFIX_RSA_SHA512 + hashBytes)
         else:
-            raise Exception("Algorithm not supported: {}".format(algo))
+            raise Exception(f"Algorithm not supported: {algo}")
         if not verify:
             raise Exception("Certificate not Signed by Provided CA Certificate Chain")