diff --git a/swh/scheduler/celery_backend/config.py b/swh/scheduler/celery_backend/config.py
--- a/swh/scheduler/celery_backend/config.py
+++ b/swh/scheduler/celery_backend/config.py
@@ -160,6 +160,8 @@
     # be json (default), pickle, yaml, msgpack, or any custom
     # serialization methods that have been registered with
     CELERY_TASK_SERIALIZER='msgpack',
+    # Result serialization format
+    CELERY_RESULT_SERIALIZER='msgpack',
     # Late ack means the task messages will be acknowledged after the task has
     # been executed, not just before, which is the default behavior.
     CELERY_ACKS_LATE=True,