diff --git a/duniterpy/documents/certification.py b/duniterpy/documents/certification.py
index bf446a64892d81fb85e3d0c01e78f2695e5da74d..1f570239e2cfd39b823a50f8d0fe8c1ee1c20562 100644
--- a/duniterpy/documents/certification.py
+++ b/duniterpy/documents/certification.py
@@ -173,7 +173,7 @@ CertTimestamp: {timestamp}
         self.signatures = []
         for key in keys:
             signing = base64.b64encode(key.signature(bytes(self.raw(), 'ascii')))
-            logging.debug("Signature : \n{0}".format(signing.decode("ascii")))
+            logging.debug("Signature : \n%s", signing.decode("ascii"))
             self.signatures.append(signing.decode("ascii"))
 
     def signed_raw(self) -> str: