diff --git a/src/_zkapauthorizer/resource.py b/src/_zkapauthorizer/resource.py
index 62c3f3ca967cc76250dcedad72fe5c3e1d957840..2a37249287e27d0f1f1dd7ab4d1abd32d8ad6a16 100644
--- a/src/_zkapauthorizer/resource.py
+++ b/src/_zkapauthorizer/resource.py
@@ -403,7 +403,7 @@ class _VoucherCollection(Resource):
             payload = loads(request.content.read())
         except Exception:
             return bad_request("json request body required").render(request)
-        if set(payload) != {"voucher"}:
+        if payload.keys() != {"voucher"}:
             return bad_request(
                 "request object must have exactly one key: 'voucher'"
             ).render(request)