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
@@ -85,7 +85,9 @@
 class TaskRouter:
     """Route tasks according to the task_queue attribute in the task class"""
     def route_for_task(self, task, args=None, kwargs=None):
-        task_class = app.tasks[task]
+        task_class = app.tasks.get(task)
+        if task_class is None:
+            return {'queue': task}
         if hasattr(task_class, 'task_queue'):
             return {'queue': task_class.task_queue}
         return None
diff --git a/swh/scheduler/celery_backend/runner.py b/swh/scheduler/celery_backend/runner.py
--- a/swh/scheduler/celery_backend/runner.py
+++ b/swh/scheduler/celery_backend/runner.py
@@ -4,6 +4,7 @@
 # See top-level LICENSE file for more information
 
 import arrow
+import celery
 from celery import group
 
 from swh.scheduler import get_scheduler, compute_nb_tasks_from
@@ -48,7 +49,11 @@
             max_queue_length = task_type['max_queue_length']
             backend_name = task_type['backend_name']
             if max_queue_length and backend_name in app.tasks:
-                queue_name = app.tasks[backend_name].task_queue
+                task = app.tasks.get(backend_name)
+                if task:
+                    queue_name = task.task_queue
+                else:
+                    queue_name = backend_name
                 queue_length = app.get_queue_length(queue_name)
                 if queue_length is None:
                     # Running without RabbitMQ (probably a test env).
@@ -77,9 +82,9 @@
             args = task['arguments']['args']
             kwargs = task['arguments']['kwargs']
 
-            celery_task = app.tasks[
-                task_types[task['type']]['backend_name']
-            ].s(*args, **kwargs)
+            celery_task = celery.Signature(
+                task_types[task['type']]['backend_name'],
+                args=args, kwargs=kwargs)
 
             celery_tasks.append(celery_task)