diff --git a/asclepias_broker/search/query.py b/asclepias_broker/search/query.py
index db87841..9fec39f 100644
--- a/asclepias_broker/search/query.py
+++ b/asclepias_broker/search/query.py
@@ -26,12 +26,6 @@ def search_factory(self, search, query_parser=None):
     from invenio_records_rest.sorter import default_sorter_factory
     search_index = search._index[0]
 
-    # TODO: make "scheme" optional?
-    for field in ('id', 'scheme', 'relation'):
-        if field not in request.values:
-            raise RESTValidationError(
-                errors=[FieldError(field, 'Required field.')])
-
     search, urlkwargs = default_facets_factory(search, search_index)
     search, sortkwargs = default_sorter_factory(search, search_index)
     for key, value in sortkwargs.items():
diff --git a/tests/api/test_relationships.py b/tests/api/test_relationships.py
index c20adf7..36c4af2 100644
--- a/tests/api/test_relationships.py
+++ b/tests/api/test_relationships.py
@@ -13,28 +13,34 @@
 from asclepias_broker.events.api import EventAPI
 
 
-def test_invalid_search_parameters(client):
+def test_optional_search_parameters(client):
     search_url = url_for('invenio_records_rest.relid_list')
 
+    # TODO: the results should not be 0
     params = {}
     resp = client.get(search_url)
-    assert resp.status_code == 400
-    assert resp.json['message'] == 'Validation error.'
-    assert resp.json['errors'][0]['field'] == 'id'
+    assert resp.status_code == 200
+    assert resp.json['hits']['total'] == 0
 
-    params['id'] = 'some-id'
+    params['id'] = 'X'
     resp = client.get(search_url, query_string=params)
-    assert resp.status_code == 400
-    assert resp.json['message'] == 'Validation error.'
-    assert len(resp.json['errors']) == 1
-    assert resp.json['errors'][0]['field'] == 'scheme'
+    assert resp.status_code == 200
+    assert resp.json['hits']['total'] == 0
 
     params['scheme'] = 'doi'
     resp = client.get(search_url, query_string=params)
-    assert resp.status_code == 400
-    assert resp.json['message'] == 'Validation error.'
-    assert len(resp.json['errors']) == 1
-    assert resp.json['errors'][0]['field'] == 'relation'
+    assert resp.status_code == 200
+    assert resp.json['hits']['total'] == 0
+
+
+def test_invalid_search_parameters(client):
+    search_url = url_for('invenio_records_rest.relid_list')
+
+    params = {}
+    params['id'] = 'some-id'
+    resp = client.get(search_url, query_string=params)
+    assert resp.status_code == 200
+    assert resp.json['hits']['total'] == 0
 
     params['relation'] = 'not-a-valid-value'
     resp = client.get(search_url, query_string=params)