Cannot delete OLT from NEM

Deleting OLT from NEM fails with the following error

Traceback (most recent call last):
File "/opt/xos/coreapi/decorators.py", line 39, in wrapper
return function(*args, **kwargs)
File "/opt/xos/coreapi/apistats.py", line 43, in wrapper
res = function(*args, **kwargs)
File "/opt/xos/coreapi/xos_grpc_api.py", line 1899, in DeleteOLTDevice
res = self.delete(model, user, request.id)
File "/opt/xos/coreapi/apihelper.py", line 447, in delete
obj.delete()
File "/opt/xos/services/volt/models.py", line 70, in delete
used_onus = [o.onu_device_id for o in self.get_volt_si()]
File "/opt/xos/services/volt/models.py", line 49, in get_volt_si
return VOLTServiceInstance.objects.all()
NameError: global name 'VOLTServiceInstance' is not defined
Exception calling application: global name 'VOLTServiceInstance' is not defined exc_info=(<type 'exceptions.NameError'>, NameError("global name 'VOLTServiceInstance' is not defined",), <traceback object at 0x7f90ad7a0518>)