diff --git a/src/_zkapauthorizer/controller.py b/src/_zkapauthorizer/controller.py
index 885d2055073e15f11e05538826739a046b77f3b4..7dc5146ddd15a11a8837a8b9af7b928069bc58b7 100644
--- a/src/_zkapauthorizer/controller.py
+++ b/src/_zkapauthorizer/controller.py
@@ -944,8 +944,8 @@ class PaymentController(object):
             self._unpaid[voucher] = self.store.now()
         else:
             self._log.error(
-                "Redeeming random tokens for a voucher ({voucher}) failed: {reason}",
-                reason=reason,
+                "Redeeming random tokens for a voucher ({voucher}) failed: {reason!r}",
+                reason=reason.value,
                 voucher=voucher,
             )
             self._error[voucher] = model_Error(