diff --git a/backend/api/models.py b/backend/api/models.py
index 084d55d..e82a41e 100644
--- a/backend/api/models.py
+++ b/backend/api/models.py
@@ -112,6 +112,6 @@ state_model = api_state.model('Recorder State', {
     'previous': fields.Nested(api_state.model('Previous Recorder State', {
         'msg': fields.String(),
         'state_ok': fields.Boolean(),
-        'time_stamp': fields.String()}, required=False)
+        'time_stamp': fields.String()}, required=False, skip_none=True)
                               )
 })