cbtransfer does not recognize a temporary folder storing 1.8.1 sqlite files as a valid source

I install 1.8.1-937-rel on a single node, create a default bucket and load some items. Then I do the following operation:

[chisheng@cen-1715 tmp]$ cp -r /opt/couchbase/var/lib/couchbase/data/ backup/.
[chisheng@cen-1715 tmp]$ ls
automake-1.11.1 backup couchbase-server-enterprise_x86_64_1.8.1-937-rel.rpm hsperfdata_root libtool-2.4.2
[chisheng@cen-1715 tmp]$ cd backup/
[chisheng@cen-1715 backup]$ ls
data
[chisheng@cen-1715 backup]$ cd data
[chisheng@cen-1715 data]$ ls
default-data isasl.pw ns_log
[chisheng@cen-1715 data]$ cd default-data/
[chisheng@cen-1715 default-data]$ ls
default default-0.mb-shm default-1.mb default-1.mb-wal default-2.mb-shm default-3.mb default-3.mb-wal default-wal
default-0.mb default-0.mb-wal default-1.mb-shm default-2.mb default-2.mb-wal default-3.mb-shm default-shm
[chisheng@cen-1715 default-data]$ cd
[chisheng@cen-1715 ~]$ cd membase-cli/
[chisheng@cen-1715 membase-cli]$ sudo mkdir /opt/couchbase/var/lib/couchbase/data/default
[chisheng@cen-1715 membase-cli]$ ./cbtransfer /tmp/backup/data/default-data couchstore-files:///opt/couchbase/var/lib/couchbase/data -b default --source-vbucket-state=active --destination-vbucket-state=active
warning: could not import couchstore module; accessing local couchstore-files:// will not work; please check your PYTHONPATH and LD_LIBRARY_PATH environment variables
error: unknown type of source: /tmp/backup/data/default-data/default

sqlite files under /tmp/backup/ can't be recognized
When I run:
$python -v
>>>import sqlite3
>>>import sqlite3.version
output: '2.4.1'