From 85a02216d8a1926f2a528e0bae64c76410a6263d Mon Sep 17 00:00:00 2001
From: Christian Grothoff <christian@grothoff.org>
Date: Mon, 6 Jul 2020 10:52:24 +0200
Subject: fix #5930

---
 src/backend/taler-merchant-httpd_private-post-orders.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/backend/taler-merchant-httpd_private-post-orders.c b/src/backend/taler-merchant-httpd_private-post-orders.c
index 06fd562b..5ccad64d 100644
--- a/src/backend/taler-merchant-httpd_private-post-orders.c
+++ b/src/backend/taler-merchant-httpd_private-post-orders.c
@@ -673,8 +673,17 @@ patch_order (struct MHD_Connection *connection,
 
 
   /* Fill in merchant information if necessary */
-  if (NULL == json_object_get (order,
+  if (NULL != json_object_get (order,
                                "merchant"))
+  {
+    GNUNET_break_op (0);
+    return TALER_MHD_reply_with_error (
+      connection,
+      MHD_HTTP_BAD_REQUEST,
+      TALER_EC_PROPOSAL_ORDER_PARSE_ERROR,
+      "'merchant' field already set, but must be provided by backend");
+  }
+  else
   {
     const char *mj = NULL;
     const char *ma = NULL;
-- 
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