error of ec2driver

2018-08-15 12:41:56.999 7646 INFO nova.virt.driver [req-098d621e-b7dd-4839-b10d-44c483d294d1 - - - - -] Loading compute driver 'ec2.EC2Driver'
2018-08-15 12:41:57.055 7646 CRITICAL nova [req-098d621e-b7dd-4839-b10d-44c483d294d1 - - - - -] TypeError: cannot concatenate 'str' and 'NoneType' objects
2018-08-15 12:41:57.055 7646 ERROR nova Traceback (most recent call last):
2018-08-15 12:41:57.055 7646 ERROR nova   File "/usr/bin/nova-compute", line 10, in <module>
2018-08-15 12:41:57.055 7646 ERROR nova     sys.exit(main())
2018-08-15 12:41:57.055 7646 ERROR nova   File "/usr/lib/python2.7/dist-packages/nova/cmd/compute.py", line 60, in main
2018-08-15 12:41:57.055 7646 ERROR nova     db_allowed=CONF.conductor.use_local)
2018-08-15 12:41:57.055 7646 ERROR nova   File "/usr/lib/python2.7/dist-packages/nova/service.py", line 218, in create
2018-08-15 12:41:57.055 7646 ERROR nova     db_allowed=db_allowed)
2018-08-15 12:41:57.055 7646 ERROR nova   File "/usr/lib/python2.7/dist-packages/nova/service.py", line 92, in __init__
2018-08-15 12:41:57.055 7646 ERROR nova     self.manager = manager_class(host=self.host, *args, **kwargs)
2018-08-15 12:41:57.055 7646 ERROR nova   File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 540, in __init__
2018-08-15 12:41:57.055 7646 ERROR nova     self.driver = driver.load_compute_driver(self.virtapi, compute_driver)
2018-08-15 12:41:57.055 7646 ERROR nova   File "/usr/lib/python2.7/dist-packages/nova/virt/driver.py", line 1626, in load_compute_driver
2018-08-15 12:41:57.055 7646 ERROR nova     virtapi)
2018-08-15 12:41:57.055 7646 ERROR nova   File "/usr/lib/python2.7/dist-packages/oslo_utils/importutils.py", line 44, in import_object
2018-08-15 12:41:57.055 7646 ERROR nova     return import_class(import_str)(*args, **kwargs)
2018-08-15 12:41:57.055 7646 ERROR nova   File "/usr/lib/python2.7/dist-packages/nova/virt/ec2/ec2driver.py", line 185, in __init__
2018-08-15 12:41:57.055 7646 ERROR nova     aws_endpoint = "ec2." + aws_region + ".amazonaws.com"
2018-08-15 12:41:57.055 7646 ERROR nova TypeError: cannot concatenate 'str' and 'NoneType' objects
2018-08-15 12:41:57.055 7646 ERROR nova
root@compute-EC2:~#