diff --git a/tempest/lib/api_schema/response/volume/volumes.py b/tempest/lib/api_schema/response/volume/volumes.py
index ffcf488..4f44526 100644
--- a/tempest/lib/api_schema/response/volume/volumes.py
+++ b/tempest/lib/api_schema/response/volume/volumes.py
@@ -21,7 +21,7 @@
     'items': {
         'type': 'object',
         'properties': {
-            'server_id': {'type': 'string', 'format': 'uuid'},
+            'server_id': {'type': ['string', 'null'], 'format': 'uuid'},
             'attachment_id': {'type': 'string', 'format': 'uuid'},
             'attached_at': parameter_types.date_time_or_null,
             'host_name': {'type': ['string', 'null']},