API RackUnitListView yields a TypeError when no device exclusion is specified

[jeremystretch]

Traceback:

File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/exception.py" in inner
  39.             response = get_response(request)

File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py" in _get_response
  187.                 response = self.process_exception_by_middleware(e, request)

File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py" in _get_response
  185.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/usr/local/lib/python2.7/dist-packages/django/views/decorators/csrf.py" in wrapped_view
  58.         return view_func(*args, **kwargs)

File "/usr/local/lib/python2.7/dist-packages/django/views/generic/base.py" in view
  68.             return self.dispatch(request, *args, **kwargs)

File "/usr/local/lib/python2.7/dist-packages/rest_framework/views.py" in dispatch
  477.             response = self.handle_exception(exc)

File "/usr/local/lib/python2.7/dist-packages/rest_framework/views.py" in handle_exception
  437.             self.raise_uncaught_exception(exc)

File "/usr/local/lib/python2.7/dist-packages/rest_framework/views.py" in dispatch
  474.             response = handler(request, *args, **kwargs)

File "/opt/netbox/netbox/dcim/api/views.py" in get
  122.             exclude = int(request.GET.get('exclude', None))

Exception Type: TypeError at /api/dcim/racks/437/rack-units/
Exception Value: int() argument must be a string or a number, not 'NoneType'