cbbackup cbb_max_mx flag causes crash

Mikes-MBP:couchbase-cli mikewied$ ./cbbackup http://127.0.0.1:9000 /tmp/backup2 -x 'cbb_max_mb=1'
Exception in thread s0:
Traceback (most recent call last):
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py", line 810, in __bootstrap_inner
self.run()
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py", line 763, in run
self._target(*self.args, **self._kwargs)
File "/Users/mikewied/couchbase/spock/couchbase-cli/pump_bfd.py", line 651, in run
self.cur['failoverlog'][(self.bucket_name(), self.node_name())])
File "/Users/mikewied/couchbase/spock/couchbase-cli/pump_bfd.py", line 99, in write_json_file
filepath = os.path.join(parent_dir, filename)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py", line 70, in join
elif path == '' or path.endswith('/'):
AttributeError: 'NoneType' object has no attribute 'endswith'