@task
    def hello():
        return "hello"

    hello.expand()

[2022-05-03 03:41:23,779] {scheduler_job.py:753} ERROR - Exception when executing SchedulerJob._run_scheduler_loop                 
Traceback (most recent call last):                                                                                                 
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/jobs/scheduler_job.py", line 736, in _execute                     
    self._run_scheduler_loop()                                                                                                     
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/jobs/scheduler_job.py", line 824, in _run_scheduler_loop          
    num_queued_tis = self._do_scheduling(session)                                                                                  
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/jobs/scheduler_job.py", line 906, in _do_scheduling               
    callback_to_run = self._schedule_dag_run(dag_run, session)                                                                     
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/jobs/scheduler_job.py", line 1148, in _schedule_dag_run           
    schedulable_tis, callback_to_run = dag_run.update_state(session=session, execute_callbacks=False)                              
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/utils/session.py", line 68, in wrapper                            
    return func(*args, **kwargs)                                                                                                   
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/models/dagrun.py", line 522, in update_state                      
    info = self.task_instance_scheduling_decisions(session)                                                                        
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/utils/session.py", line 68, in wrapper                            
    return func(*args, **kwargs)                                                                                                   
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/models/dagrun.py", line 661, in task_instance_scheduling_decisions
    session=session,                                                                                                               
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/models/dagrun.py", line 714, in _get_ready_tis                    
    expanded_tis, _ = schedulable.task.expand_mapped_task(self.run_id, session=session)                                            
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/models/mappedoperator.py", line 609, in expand_mapped_task        
    operator.mul, self._resolve_map_lengths(run_id, session=session).values()                                                      
TypeError: reduce() of empty sequence with no initial value