UserBackedStorageObject creation failed

[platoxie]

targetcli
/backstores/user:rbd> create cfgstring=rbd/imge1 name=rbd0 size=128m
UserBackedStorageObject creation failed.

targetcli version 2.1.fb47
rtslib-fb version 2.1.64
kernel version 4.13.0-0.rc6

vi ~/.targetcli/log.txt
46 get_node() Looking for path '.'
[DEBUG] 2017-08-29 17:27:19 /usr/lib/python2.7/site-packages/configshell_fb-1.1.23-py2.7.egg/configshell_fb/node.py:1398 execute_command() Executing command create with pparams [] and kparams {'cfgstring': 'rbd/imge1', 'name': 'rbd0', 'size': '128m'}.
[DEBUG] 2017-08-29 17:27:19 /usr/lib/python2.7/site-packages/configshell_fb-1.1.23-py2.7.egg/configshell_fb/node.py:1445 assert_params() Min params: 3
[DEBUG] 2017-08-29 17:27:19 /usr/lib/python2.7/site-packages/configshell_fb-1.1.23-py2.7.egg/configshell_fb/node.py:1446 assert_params() Max params: 5
[DEBUG] 2017-08-29 17:27:19 /usr/lib/python2.7/site-packages/configshell_fb-1.1.23-py2.7.egg/configshell_fb/node.py:1447 assert_params() Required params: name, size, cfgstring
[DEBUG] 2017-08-29 17:27:19 /usr/lib/python2.7/site-packages/configshell_fb-1.1.23-py2.7.egg/configshell_fb/node.py:1448 assert_params() Optional params: wwn, hw_max_sectors
[DEBUG] 2017-08-29 17:27:19 /usr/lib/python2.7/site-packages/configshell_fb-1.1.23-py2.7.egg/configshell_fb/node.py:1449 assert_params() Got 3 standard params.
[DEBUG] 2017-08-29 17:27:19 /usr/lib/python2.7/site-packages/configshell_fb-1.1.23-py2.7.egg/configshell_fb/node.py:1450 assert_params() Got 0 extended params.
[DEBUG] 2017-08-29 17:27:19 /usr/lib/python2.7/site-packages/configshell_fb-1.1.23-py2.7.egg/configshell_fb/node.py:1451 assert_params() Variable positional params: None
[DEBUG] 2017-08-29 17:27:19 /usr/lib/python2.7/site-packages/configshell_fb-1.1.23-py2.7.egg/configshell_fb/node.py:1452 assert_params() Variable keyword params: None
[ERROR] 2017-08-29 17:27:19 UserBackedStorageObject creation failed.