cbbackup: The differential backup fails for data backed up from 2.5 build

steps to reproduce:

• Install latest 2.5.1 build (in this case used 1094 release)
• Add 80k items on setup (current setup optional: 2 nodes and 1 default bucket)
• Take backup in dir /tmp/backup
• Uninstall the current build and install latest 3.0.0 build (in this case installed-859 rel)
  -take a full backup in different dir /tmp/backupnew
• Add a new item to documents
• Take differential backup
  cbbackup throws error and hangs:
  captured stdout:

[root@centos-64-x64 bin]# ./cbbackup http://Administrator:password@172.23.106.71:8091 /tmp/backup
Exception in thread w0:
Traceback (most recent call last):
File "/usr/lib64/python2.6/threading.py", line 532, in __bootstrap_inner
self.run()
File "/usr/lib64/python2.6/threading.py", line 484, in run
self._target(*self.args, **self._kwargs)
File "/opt/couchbase/lib/python/pump.py", line 286, in run_worker
source_map, sink_map, self.ctl, curx).run()
File "/opt/couchbase/lib/python/pump.py", line 344, in run
rv_batch, batch = self.source.provide_batch()
File "/opt/couchbase/lib/python/pump_upr.py", line 89, in provide_batch
rv, upr_conn = self.get_upr_conn()
File "/opt/couchbase/lib/python/pump_upr.py", line 366, in get_upr_conn
self.setup_upr_streams()
File "/opt/couchbase/lib/python/pump_upr.py", line 453, in setup_upr_streams
reverse=True)
File "/opt/couchbase/lib/python/pump_upr.py", line 452, in <lambda>
key=lambda tup: tup[1],
TypeError: 'int' object is unsubscriptable

Exception in thread w1:
Traceback (most recent call last):
File "/usr/lib64/python2.6/threading.py", line 532, in __bootstrap_inner
self.run()
File "/usr/lib64/python2.6/threading.py", line 484, in run
self._target(*self.args, **self._kwargs)
File "/opt/couchbase/lib/python/pump.py", line 286, in run_worker
source_map, sink_map, self.ctl, curx).run()
File "/opt/couchbase/lib/python/pump.py", line 344, in run
rv_batch, batch = self.source.provide_batch()
File "/opt/couchbase/lib/python/pump_upr.py", line 89, in provide_batch
rv, upr_conn = self.get_upr_conn()
File "/opt/couchbase/lib/python/pump_upr.py", line 366, in get_upr_conn
self.setup_upr_streams()
File "/opt/couchbase/lib/python/pump_upr.py", line 453, in setup_upr_streams
reverse=True)
File "/opt/couchbase/lib/python/pump_upr.py", line 452, in <lambda>
key=lambda tup: tup[1],
TypeError: 'int' object is unsubscriptable