diff --git a/gregui/mailutils.py b/gregui/mailutils.py
index 435224cf39972e618cce0b2113d418a956856888..1079aeba687a618d00e85bc97608d38bfb5e27c5 100644
--- a/gregui/mailutils.py
+++ b/gregui/mailutils.py
@@ -81,7 +81,7 @@ def send_invite_mail(link: InvitationLink) -> Optional[str]:
     sponsor = link.invitation.role.sponsor
     if not sponsor:
         logger.warning(
-            "Unable to determine sponsor for invitation link with ID: {link.id}"
+            "Unable to determine sponsor for invitation link with ID: {%s}" % link.id
         )
         return None