freezer-agent report "Private key file is encrypted" error when using ssh storage

backup execution with ssh storage:
zhusz@zhusz-HP-Pro-2080-Microtower-PC:~/freezer$ sudo freezer-agent --path-to-backup ../freezer-api --container /home/openstack/freezer-api-backup --backup-name freezer-api --storage ssh --ssh-username openstack --ssh-key /home/zhusz/.ssh/id_rsa --ssh-host 10.118.202.201 --debug

The following error occurs:
[*] Critical Error: Private key file is encrypted

logs:
2016-07-25 17:09:41.648 17551 INFO freezer.main [-] log file at /home/zhusz/.freezer/freezer.log
2016-07-25 17:09:41.649 17551 DEBUG paramiko.transport [-] starting thread (client mode): 0x5084c10L _log /usr/local/lib/python2.7/dist-packages/paramiko/transport.py:1546
2016-07-25 17:09:41.650 17551 DEBUG paramiko.transport [-] Local version/idstring: SSH-2.0-paramiko_2.0.0 _log /usr/local/lib/python2.7/dist-packages/paramiko/transport.py:1546
2016-07-25 17:09:41.665 17551 DEBUG paramiko.transport [-] Remote version/idstring: SSH-2.0-OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.7 _log /usr/local/lib/python2.7/dist-packages/paramiko/transport.py:1546
2016-07-25 17:09:41.665 17551 INFO paramiko.transport [-] Connected (version 2.0, client OpenSSH_6.6.1p1)
...
2016-07-25 17:09:41.667 17551 DEBUG paramiko.transport [-] Kex agreed: diffie-hellman-group1-sha1 _log /usr/local/lib/python2.7/dist-packages/paramiko/transport.py:1546
2016-07-25 17:09:41.667 17551 DEBUG paramiko.transport [-] Cipher agreed: aes128-ctr _log /usr/local/lib/python2.7/dist-packages/paramiko/transport.py:1546
2016-07-25 17:09:41.667 17551 DEBUG paramiko.transport [-] MAC agreed: hmac-sha2-256 _log /usr/local/lib/python2.7/dist-packages/paramiko/transport.py:1546
2016-07-25 17:09:41.668 17551 DEBUG paramiko.transport [-] Compression agreed: none _log /usr/local/lib/python2.7/dist-packages/paramiko/transport.py:1546
2016-07-25 17:09:42.329 17551 DEBUG paramiko.transport [-] kex engine KexGroup1 specified hash_algo <built-in function openssl_sha1> _log /usr/local/lib/python2.7/dist-packages/paramiko/transport.py:1546
2016-07-25 17:09:42.330 17551 DEBUG paramiko.transport [-] Switch to new keys ... _log /usr/local/lib/python2.7/dist-packages/paramiko/transport.py:1546
2016-07-25 17:09:42.332 17551 DEBUG paramiko.transport [-] Adding ssh-rsa host key for 10.118.202.201: 86f772705715dfa9a03251bd2a28dbd4 _log /usr/local/lib/python2.7/dist-packages/paramiko/transport.py:1546
2016-07-25 17:09:42.333 17551 CRITICAL freezer.main [-] [*] Critical Error: Private key file is encrypted
2016-07-25 17:09:42.431 17551 DEBUG paramiko.transport [-] EOF in transport thread _log /usr/local/lib/python2.7/dist-packages/paramiko/transport.py:1546