如何使用Airflows SSHOperator连接到ssh？

Question

我正在尝试使用SSHOperator通过气流连接到ec2实例，但是我的dag失败并显示以下错误消息：

[2020-05-24 06:17:04,191] {base_task_runner.py:98} INFO - Subtask:   File "/usr/local/lib/python3.6/site-packages/airflow/contrib/operators/ssh_operator.py", line 146, in execute
[2020-05-24 06:17:04,191] {base_task_runner.py:98} INFO - Subtask:     raise AirflowException("SSH operator error: {0}".format(str(e)))
[2020-05-24 06:17:04,191] {base_task_runner.py:98} INFO - Subtask: airflow.exceptions.AirflowException: SSH operator error: 'NoneType' object has no attribute 'exec_command'

这是我的源代码：

from airflow.contrib.operators.ssh_operator import SSHOperator
...

t1 = BashOperator(
    task_id='print_date',
    bash_command='date',
    dag=DAG)

t2 = SSHOperator(
    ssh_conn_id='my_ec2_connection',
    task_id='ssh_operator',
    command='ls',
    dag=DAG)

t1 >> t2

我已经在UI中使用ssh连接设置了pem文件，并且还对pem密钥进行了ssh-add。