primitiv: [FreeBSD] Python runtime error: "Default object is null."

Steps to reproduce:

Install primitiv in FreeBSD 11.1 by the following commands (root):

pkg install protobuf cmake git googletest python3
python3 -m ensurepip
pip3 install numpy cython
git clone https://github.com/odashi/primitiv.git
cd primitiv
cmake . -DPRIMITIV_BUILD_TESTS=ON
make
make test
make install
cd python-primitiv
env CPLUS_INCLUDE_PATH=/usr/local/include ./setup.py build

The following python code throws an error:

>>> from primitiv import *
>>> dev = devices.Naive()
>>> Device.set_default(dev)
>>> g = Graph()
>>> Graph.set_default(g)
>>> dev == Device.get_default()
True
>>> g == Graph.get_default()
True
>>> node = operators.ones([2,3])               
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "primitiv/_operator.pyx", line 226, in primitiv._operator._operators.ones
    return wrapNode(op_ones[CppNode](normShape(shape).wrapped, device.wrapped[0]))
RuntimeError: /root/primitiv/primitiv/mixins.h: 69: Default object is null.