diff --git a/gregui/mailutils.py b/gregui/mailutils.py
index 1079aeba687a618d00e85bc97608d38bfb5e27c5..e51a31ece85104a8ab82381f09f12c3a3d36933b 100644
--- a/gregui/mailutils.py
+++ b/gregui/mailutils.py
@@ -74,14 +74,14 @@ def send_invite_mail(link: InvitationLink) -> Optional[str]:
     email_address = link.invitation.role.person.private_email
     if not email_address:
         logger.warning(
-            "No e-mail address found for invitation link with ID: {%s}" % link.id
+            "No e-mail address found for invitation link with ID: {%s}", link.id
         )
         return None
 
     sponsor = link.invitation.role.sponsor
     if not sponsor:
         logger.warning(
-            "Unable to determine sponsor for invitation link with ID: {%s}" % link.id
+            "Unable to determine sponsor for invitation link with ID: {%s}", link.id
         )
         return None