diff --git a/src/_zkapauthorizer/controller.py b/src/_zkapauthorizer/controller.py
index f6af30064c04fcf777152c7634d78b0830299040..16c2f53dd29c50f7073b0d6294996b6be2b234fa 100644
--- a/src/_zkapauthorizer/controller.py
+++ b/src/_zkapauthorizer/controller.py
@@ -177,4 +177,8 @@ class PaymentController(object):
         )
 
     def _redeemSuccess(self, voucher, passes):
+        """
+        Update the database state to reflect that a voucher was redeemed and to
+        store the resulting passes.
+        """
         self.store.insert_passes_for_voucher(voucher, passes)