diff --git a/src/certificate.py b/src/certificate.py
index ddd126852..17a6350f3 100644
--- a/src/certificate.py
+++ b/src/certificate.py
@@ -588,7 +588,7 @@ def _prepare_certificate_signing_request(domain, key_file, output_folder):
                 crypto.X509Extension(
                     b"subjectAltName",
                     False,
-                    (", ".join(["DNS:{sub}.{domain}" for sub in sanlist])).encode("utf-8"),
+                    (", ".join([f"DNS:{sub}.{domain}" for sub in sanlist])).encode("utf-8"),
                 )
             ]
         )