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
@@ -98,7 +98,8 @@
         Arguments:
           queue_name: name of the queue to check
 
-        Returns a dictionary raw from the RabbitMQ management API.
+        Returns a dictionary raw from the RabbitMQ management API;
+        or `None` if the current configuration does not use RabbitMQ.
 
         Interesting keys:
          - consumers (number of consumers for the queue)
@@ -110,6 +111,9 @@
         """
 
         conn_info = self.connection().info()
+        if conn_info['transport'] == 'memory':
+            # We're running in a test environment, without RabbitMQ.
+            return None
         url = 'http://{hostname}:{port}/api/queues/{vhost}/{queue}'.format(
             hostname=conn_info['hostname'],
             port=conn_info['port'] + 10000,
@@ -125,7 +129,9 @@
 
     def get_queue_length(self, queue_name):
         """Shortcut to get a queue's length"""
-        return self.get_queue_stats(queue_name)['messages']
+        stats = self.get_queue_stats(queue_name)
+        if stats:
+            return stats['messages']
 
 
 INSTANCE_NAME = os.environ.get(CONFIG_NAME_ENVVAR)
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
@@ -50,8 +50,12 @@
             if max_queue_length and backend_name in app.tasks:
                 queue_name = app.tasks[backend_name].task_queue
                 queue_length = app.get_queue_length(queue_name)
-                num_tasks = min(max_queue_length - queue_length,
-                                MAX_NUM_TASKS)
+                if queue_length is None:
+                    # Running without RabbitMQ (probably a test env).
+                    num_tasks = MAX_NUM_TASKS
+                else:
+                    num_tasks = min(max_queue_length - queue_length,
+                                    MAX_NUM_TASKS)
             else:
                 num_tasks = MAX_NUM_TASKS
             if num_tasks > 0: