No BaseVirtualMachine subclass defined with the attributes for OpenStack

I am trying to run benchmarks with perfkit on My OpenStack Environment. When I run './pkb.py --cloud=OpenStack --machine_type=m1.medium --openstack_network=private --benchmarks=ping --image=xenial', I am getting "SubclassNotFoundError: No BaseVirtualMachine subclass defined with the attributes: {'OS_TYPE': 'ubuntu1604', 'CLOUD': 'OpenStack'}" error. Full logs are below:
ubuntu@xenialperfkit:~/PerfKitBenchmarker$ ./pkb.py --cloud=OpenStack --machine_type=m1.medium --openstack_network=private --benchmarks=ping --image=xenial
2019-03-05 07:15:33,185 6e735b77 MainThread INFO Verbose logging to: /tmp/perfkitbenchmarker/runs/6e735b77/pkb.log
2019-03-05 07:15:33,186 6e735b77 MainThread INFO PerfKitBenchmarker version: v1.12.0-1027-g00463d2
2019-03-05 07:15:33,186 6e735b77 MainThread INFO Flag values:
--machine_type=m1.medium
--cloud=OpenStack
--openstack_network=private
--benchmarks=ping
--image=cirros-0.3.4-x86_64-uec
2019-03-05 07:15:33,357 6e735b77 MainThread INFO Setting --max_concurrent_threads=200.
2019-03-05 07:15:33,382 6e735b77 MainThread WARNING The key "flags" was not in the default config, but was in user overrides. This may indicate a typo.
2019-03-05 07:15:33,495 6e735b77 MainThread ping(1/1) INFO Provisioning resources for benchmark ping
2019-03-05 07:15:33,495 6e735b77 MainThread ping(1/1) ERROR Error during benchmark ping
Traceback (most recent call last):
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/pkb.py", line 726, in RunBenchmark
DoProvisionPhase(spec, detailed_timer)
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/pkb.py", line 520, in DoProvisionPhase
spec.ConstructVirtualMachines()
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/benchmark_spec.py", line 404, in ConstructVirtualMachines
vms = self.ConstructVirtualMachineGroup(group_name, group_spec)
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/benchmark_spec.py", line 343, in ConstructVirtualMachineGroup
vm = self._CreateVirtualMachine(group_spec.vm_spec, os_type, cloud)
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/benchmark_spec.py", line 659, in _CreateVirtualMachine
vm_class = virtual_machine.GetVmClass(cloud, os_type)
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/virtual_machine.py", line 75, in GetVmClass
CLOUD=cloud, OS_TYPE=os_type)
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/resource.py", line 47, in GetResourceClass
(base_class.__name__, kwargs))
SubclassNotFoundError: No BaseVirtualMachine subclass defined with the attributes: {'OS_TYPE': 'ubuntu1604', 'CLOUD': 'OpenStack'}
2019-03-05 07:15:33,496 6e735b77 MainThread ping(1/1) ERROR Exception running benchmark
Traceback (most recent call last):
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/pkb.py", line 874, in RunBenchmarkTask
RunBenchmark(spec, collector)
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/pkb.py", line 726, in RunBenchmark
DoProvisionPhase(spec, detailed_timer)
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/pkb.py", line 520, in DoProvisionPhase
spec.ConstructVirtualMachines()
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/benchmark_spec.py", line 404, in ConstructVirtualMachines
vms = self.ConstructVirtualMachineGroup(group_name, group_spec)
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/benchmark_spec.py", line 343, in ConstructVirtualMachineGroup
vm = self._CreateVirtualMachine(group_spec.vm_spec, os_type, cloud)
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/benchmark_spec.py", line 659, in _CreateVirtualMachine
vm_class = virtual_machine.GetVmClass(cloud, os_type)
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/virtual_machine.py", line 75, in GetVmClass
CLOUD=cloud, OS_TYPE=os_type)
File "/home/ubuntu/PerfKitBenchmarker/perfkitbenchmarker/resource.py", line 47, in GetResourceClass
(base_class.__name__, kwargs))
SubclassNotFoundError: No BaseVirtualMachine subclass defined with the attributes: {'OS_TYPE': 'ubuntu1604', 'CLOUD': 'OpenStack'}
2019-03-05 07:15:33,497 6e735b77 MainThread ping(1/1) ERROR Benchmark 1/1 ping (UID: ping0) failed. Execution will continue.
2019-03-05 07:15:33,497 6e735b77 MainThread ping(1/1) INFO Benchmark run statuses:
-------------------------------------
Name UID Status Failed Substatus
-------------------------------------
ping ping0 FAILED
-------------------------------------
Success rate: 0.00% (0/1)
2019-03-05 07:15:33,497 6e735b77 MainThread ping(1/1) INFO Complete logs can be found at: /tmp/perfkitbenchmarker/runs/6e735b77/pkb.log
2019-03-05 07:15:33,497 6e735b77 MainThread ping(1/1) INFO Completion statuses can be found at: /tmp/perfkitbenchmarker/runs/6e735b77/completion_statuses.json

I am not sure this is bug or not. Maybe I am missing something. Can you help me for solve this issue ?