你好，我ubuntu上刚安装好mujoco和mujoco_py,在测试的时候，遇到这个错误

xml_path = os.path.join(mj_path, 'model', 'humanoid.xml')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/posixpath.py", line 80, in join
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not tuple