Failed to delete AutoScallingGroup with ERROR said: format requires a mapping

The Background:
I created a stack "tsing" with heat 6.0.0. There is an AutoScalingGroup named "asg". Then I used "heat stack-delete" to delete this stack.

The Error log:
492 2017-02-23 09:10:24.089 260086 DEBUG heat.engine.scheduler [-] Task destroy running step /usr/lib/python2.7/site-packages/heat/engine/scheduler.py:216
493 2017-02-23 09:10:24.097 260086 INFO heat.engine.resource [-] DELETE: AutoScalingResourceGroup "asg" [6acc9c13-c0a2-486a-9f29-b0b267a51a69] Stack "tsing" [7a1b8bc4-8da5-4c48-beca-fc90ade9c70a]
494 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource Traceback (most recent call last):
495 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resource.py", line 704, in _action_recorder
496 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource yield
497 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resource.py", line 1479, in delete
498 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource yield self.action_handler_task(action, *action_args)
499 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/scheduler.py", line 313, in wrapper
500 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource step = next(subtask)
501 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resource.py", line 749, in action_handler_task
502 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource while not check(handler_data):
503 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resources/stack_resource.py", line 463, in check_delete_complete
504 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource return self._check_status_complete(self.DELETE)
505 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resources/stack_resource.py", line 390, in _check_status_complete
506 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource action=action)
507 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource ResourceFailure: TypeError: resources.asg.resources.ttato5exw3ss.resources.member: format requires a mapping
508 2017-02-23 09:10:24.097 260086 ERROR heat.engine.resource
509 2017-02-23 09:10:24.147 260086 DEBUG heat.engine.scheduler [-] Task destroy cancelled cancel /usr/lib/python2.7/site-packages/heat/engine/scheduler.py:241
510 2017-02-23 09:10:24.148 260086 DEBUG heat.engine.scheduler [-] Task destroy cancelled cancel /usr/lib/python2.7/site-packages/heat/engine/scheduler.py:241
511 2017-02-23 09:10:24.149 260086 DEBUG heat.engine.scheduler [-] Task destroy cancelled cancel /usr/lib/python2.7/site-packages/heat/engine/scheduler.py:241
512 2017-02-23 09:10:24.149 260086 DEBUG heat.engine.scheduler [-] Task destroy cancelled cancel /usr/lib/python2.7/site-packages/heat/engine/scheduler.py:241
513 2017-02-23 09:10:24.150 260086 DEBUG heat.engine.scheduler [-] Task destroy cancelled cancel /usr/lib/python2.7/site-packages/heat/engine/scheduler.py:241
514 2017-02-23 09:10:24.151 260086 DEBUG heat.engine.scheduler [-] Task destroy cancelled cancel /usr/lib/python2.7/site-packages/heat/engine/scheduler.py:241
515 2017-02-23 09:10:24.152 260086 DEBUG heat.engine.scheduler [-] Task destroy cancelled cancel /usr/lib/python2.7/site-packages/heat/engine/scheduler.py:241
516 2017-02-23 09:10:24.156 260086 DEBUG heat.engine.service [-] Jeffrey: func = <function _dispatch at 0x6e3e488> _start_with_trace /usr/lib/python2.7/site-packages/heat/engine/service.py:117
517 2017-02-23 09:10:24.175 260086 DEBUG oslo_messaging._drivers.amqpdriver [-] CAST unique_id: 03940a8f2e154eab9c7fcb47fa033d22 NOTIFY exchange 'heat' topic 'notifications.error' _send /usr/lib/python2.7/site-packages/oslo_messag ing/_drivers/amqpdriver.py:438
518 2017-02-23 09:10:24.205 260086 INFO heat.engine.stack [-] Stack DELETE FAILED (tsing): Resource DELETE failed: TypeError: resources.asg.resources.ttato5exw3ss.resources.member: format requires a mapping
519 2017-02-23 09:10:24.206 260086 DEBUG heat.engine.stack [-] Jeffrey: values = {'action': 'DELETE', 'status': 'FAILED', 'status_reason': u'Resource DELETE failed: TypeError: resources.asg.resources.ttato5exw3ss.resources.member: format requires a mapping'} _persist_state /usr/lib/python2.7/site-packages/heat/engine/stack.py:882

Thanks,
Jeffrey