diff --git a/iceprod/rest/handlers/config.py b/iceprod/rest/handlers/config.py
index 52bda1c7..9e81c7dd 100644
--- a/iceprod/rest/handlers/config.py
+++ b/iceprod/rest/handlers/config.py
@@ -1,5 +1,6 @@
 import logging
 import json
+import copy
 
 from jsonschema.exceptions import ValidationError
 import tornado.web
@@ -79,11 +80,11 @@ async def put(self, dataset_id):
         elif data['dataset_id'] != dataset_id:
             raise tornado.web.HTTPError(400, reason='dataset_id mismatch')
         try:
-            c = Config(data)
+            c = Config(copy.deepcopy(data))
             c.fill_defaults()
             c.validate()
         except ValidationError as e:
-            raise tornado.web.HTTPError(400, reason=str(e))
+            raise tornado.web.HTTPError(400, reason=str(e).split('\n', 1)[0])
         except Exception:
             logger.warning('unknown config validation error', exc_info=True)
             raise tornado.web.HTTPError(400, reason='unknown validation error')
diff --git a/tests/rest/config_test.py b/tests/rest/config_test.py
index c8ddeeab..b6182202 100644
--- a/tests/rest/config_test.py
+++ b/tests/rest/config_test.py
@@ -1,6 +1,8 @@
 import pytest
 import requests.exceptions
 
+from iceprod.core.config import Config
+
 
 async def test_rest_config_err(server):
     client = server(roles=['system'])
@@ -14,7 +16,22 @@ async def test_rest_config(server):
     data = {
         'name': 'foo'
     }
-    await client.request('PUT', '/config/bar', data)
     
+    with pytest.raises(requests.exceptions.HTTPError) as exc_info:
+        await client.request('PUT', '/config/bar', data)
+    assert exc_info.value.response.status_code == 400
+    assert 'required property' in exc_info.value.response.text
+
+    data = {
+        'tasks': [{
+            'name': 'task',
+            'trays': [{
+                'modules': [{
+                }],
+            }],
+        }],
+    }
+    await client.request('PUT', '/config/bar', data)
+
     ret = await client.request('GET', '/config/bar')
     assert ret == data